#include <iostream>

using namespace std;

int main()
{
	cout << "Input the number of real numeric: ";
	int num = 0;
	// перехват исключения, если введено не число
	cin.exceptions(istream::failbit | istream::badbit);
	try
	{
		cin >> num;
	}
	catch (...)
	{
		cout << "Input is not a real number" << endl;
		return -1;
	}

	int input, sum = 0, devel = num;
	do
	{
		cout << "Input real numeric: ";
		try
		{
			cin >> input;
		}
		catch (...)
		{
			cout << "Input is not a real number" << endl;
			return -1;
		}
		sum += input;
	}
	while (--num > 0);

	cout << "Average: " << sum / devel << endl;
	return 0;
}

Input the number of real numeric: 3
Input real numeric: 1
Input real numeric: 3
Input real numeric: 5
Average: 3