#include <iostream>
#include <iomanip>
using namespace std;

int main()
{
	int n;
	float sum = 0;
	do
	{
		cout << "Enter n: ";
		cin >> n;
		if (n < 0)
		{
			cout << "Enter again!" << endl;
		}
	} while (n < 0);

	for (int i = 1; i <= n; i++)
	{
		sum = sum + (1.0 / (i * (i + 1)));
	}
	cout << "\nS(n) = 1 / 1 * 2 + 1 / 2 * 3 + ... + 1 / n * (n + 1) = " << setprecision(3) << sum << endl;

	cout << endl;
	system("pause");
	return 0;
}