declare ALL variables at the beginning of the program to make the program easier to read and understand

#include<iostream>

#include<cmath>

using namespace std;

int main(){

    float billAmount, inputAmount, currentInput, changeToBeReturned;         
    int choice=0;
    char lightOrRegular;
    string item="";
    const float WATER=1.25, LEMONADE=1.35, COLA=1.70, ROOTBEAR=1.55;
    //display the menu
    cout << "1- Bottled-water    $1.25" << endl;
    cout << "2- Lemonade         $1.35" <<endl;
    cout << "3- Cola             $1.70" << endl;
    cout << "4- Root-bear        $1.55" << endl;
    cout << "5- Exit" << endl;
    
    cout << "Enter your choice: ";
    cin >> choice;
    
    //this loop iterates if choice greater than 5
    while(choice>5){
        cout << "You entered an invalid choice! Try again" << endl;
        cout << "Enter your choice: ";
        cin >> choice;
    }
    
    if(choice!=5){   //if you don't want to exit
        
        if(choice==1){
            billAmount=WATER;                       //billAmount is assigned to WATER value
            item="Bottled-water" ;                  //item is assigned with Bottled-water
        } 
        else{
            
            cout << "Enter L for Light or R for Regular: ";
            cin >> lightOrRegular;
            
            //this loop iterates till you enter a valid choice Regular or Light
            while(lightOrRegular!='L' && lightOrRegular!='R'){
                cout << "You entered an invalid choice! Try again" << endl;
                cout << "Enter L for Light or R for Regular: ";
                cin >> lightOrRegular;
            }
            
            if(choice==2){
                billAmount=LEMONADE;                //billAmount is assigned to LEMONADE value
                if(lightOrRegular=='L')
                    item="Light Lemonade";
                else
                    item="Regular Lemonade";
            }
            
            else if(choice==3){
                billAmount=COLA;                    //billAmount is assigned with COLA value
                if(lightOrRegular=='L')
                    item="Light Cola";
                else
                    item="Regular Cola";
            }
            else{
                billAmount=ROOTBEAR;
                if(lightOrRegular=='L')
                    item="Light RootBear";
                else
                    item="Regular RootBear";
            }
            
            //This loop iterates till you input the amount greater than bill
            while(inputAmount<billAmount && currentInput!=-1){
                cout << "Enter " << float(billAmount-inputAmount) << "amount to proceed or -1 to quit" << endl;
                cin >> currentInput;
                if(currentInput!=-1)
                    inputAmount+=currentInput;
            }
            if(inputAmount>=billAmount){
                //calculate change to be returned
                changeToBeReturned=inputAmount-billAmount;
                cout << "Here is your " << item << endl;
                cout << "Paid:\t\t$"<< inputAmount << endl;
                cout << "Change Back:\t$ " << changeToBeReturned << endl;
                
                //finding number of bills
                //fmod is used to operate modulus operation on float numbers
                //displaying bills only if that category is greater than 0
                if(changeToBeReturned/5 >0){
                    cout << "\t\t" << (int)changeToBeReturned/5 << " Five Dollar Bill" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,5);        
                }
                if(changeToBeReturned/1 >0){
                    cout << "\t\t"  << (int)changeToBeReturned/1 << " Dollar Bill" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,1);
                }
                if((int)(changeToBeReturned/0.25) >0){
                    cout << "\t\t"  << (int)(changeToBeReturned/0.25) << " Quarter" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,0.25);
                }
                if((int)(changeToBeReturned/0.1) >0){
                    cout << "\t\t"  << (int)(changeToBeReturned/0.1) << " Dimen" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,0.1);
                }
                if((int)(changeToBeReturned/0.05) >0){
                    cout << "\t\t"  << (int)(changeToBeReturned/0.05) << " Nickel" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,0.05);
                }
                if((int)(changeToBeReturned/0.01) >0){
                    cout << "\t\t"  << (int)(changeToBeReturned/0.01) << " Penny" << endl;
                    changeToBeReturned = fmod(changeToBeReturned,0.01);
                }
            }
        }
    }
    else{
        cout << "Goodbye!" << endl;         //if 5 is entered
    }
    return 0;

}