the first task of final exam

get pmt provided r n and pv

//  main.cpp
//  Final_Nanxing_Task1
//
//  Created by Nanxing LAI on 2019/1/20.
//  Copyright © 2019 Mac. All rights reserved.

// this is the code written in Mac system, I'm not sure whether you can run it successfully in your C++, if not, please let me know it
// my email is lainanxing94@163.com. 
// I will show the table my my coding result. 
// thank you. Natalia, I learnt a lot from your lectures. 

#include<iostream>
#include<cmath>
#include<string>
#include<iomanip>


using namespace std;

//the check of interger number, only for the number of periods
int inputInteger(string prompt){
    cout << prompt;
    int n;
    cin >> n;
    
    while(n < 1){
        cout << "invalid, " << prompt;
        cin >> n;
    }
    return n;
}
// the test of double value number, which inluding r, pmt and pv
double inputDouble(string prompt){
    cout << prompt;
    double value;
    cin >> value;
    
    while(value < 0){
        cout << "invalid, " << prompt;
        cin >> value;
    }
    return value;
}



int main()
{
    double r,pv,pmt,a,b;
    int n;
    // providing values
    n = inputInteger("please input number of periods: ");
    r = inputDouble("please input periodic interest rate: ");
    pv = inputDouble("please input principal: ");
    
    a=pow(1+r,n);
    b=pv*r;
    pmt=b*(1+1/(a-1));
    
    cout<<"the pmt is "<<pmt<<endl;
    
    return 0;
}