I need the program to not take number less then 0

[Mathew Vanlaningham]

I need the program to bring up a error if you enter a number less then 0
this is my code below.

#include "stdafx.h"
using namespace System;
#include <iostream>
#include <iomanip>
using namespace std;

int main()
{

double usDollar = 0.0;
char countryNum =' ';
double exchanged = 0.0;

cout << "Mathew Vanlaningham" <<endl;

cout << "Enter U.S Dollars: $";
cin >> usDollar;

cout << "1 For Canada Dollar" <<endl;
cout << "2 For Eurozine Euro" <<endl;
cout << "3 For India Rupee" <<endl;
cout << "4 For Japan Yen" <<endl;
cout << "5 For Mexico Peso" <<endl;
cout << "6 For South Africa Rand" <<endl;
cout << "7 For United Kingdom Pound" <<endl;

cout << "Pick 1-7 to find the exchange from U.S. Dollar to that Currency: ";
cin >>countryNum;

switch(countryNum)
{
case '1':
exchanged = usDollar * 1.01615;
cout << " " << exchanged << " in Canada Dollar" <<endl;
break;
case '2':
exchanged = usDollar * .638490;
cout << " " << exchanged << " in Eurozone Euro" <<endl;
break;
case '3':
exchanged = usDollar * 40.1798;
cout << " " << exchanged << " in India Rupee" <<endl;
break;
case '4':
exchanged = usDollar * 104.390;
cout << " " << exchanged << " in Japan Yen" <<endl;
break;
case '5':
exchanged = usDollar * 10.4613;
cout << " " << exchanged << " in Mexico Peso" <<endl;
break;
case '6':
exchanged = usDollar * 7.60310;
cout << " " << exchanged << " in South Africa Rand" <<endl;
break;
case '7':
exchanged = usDollar * .504285;
cout << " " << exchanged << " United Kingdom Pound" <<endl;
break;
default:
cout << "Invalid Country Number" <<endl;
}

system("pause");
return 0;
}

[MiiNiPaa]

cout << "Enter U.S Dollars: $";
cin >> usDollar;
while(usDollar <= 0) {
    cout << "Amount should ne > 0!\nEnter again: ";
    cin >> usDollar;
}

[Mathew Vanlaningham]

Thanks for the help!