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

using std::cout;
using std::cin;
using std::endl;
using std::string;

string isItANumber(string input="")
{

	int count = 0;
	bool runAgain = true;

	while(runAgain)
	{
		if(count == 0)
		{
			for (int i = 0; i < input.length(); ++i) {
				if(!(isdigit(input[i])))
				{
					// Let runAgain stay true
					cout << "One of the characters is not a digit" << endl;
					break;
				}
				else
				{
					runAgain = false;
				}
			}
			count++;
		}
		else
		{
			cout << "Error! Please enter a number: " << endl;

			// Put the input in input
			getline(cin, input);

			// Check again
			for (int i = 0; i < input.length(); ++i) {
				if(!(isdigit(input[i])))
				{
					// Let runAgain stay true
					break;
				}
				else
				{
					runAgain = false;
				}
			}

		}


	}

	return input;
}





int main() {

	string input;

	cout << "Please enter a number: " << endl;
	getline(cin, input);

	isItANumber(input);


	return 0;
}