#include <iostream>

using namespace std;

long double function(int n);
int n;

int main() {
while(1) {
    cout << "enter n for computing F(n) (-1 to exit): ";
    cin >> n;
    cout << 2*function(n) << "\n";
    if (n == -1) 
    break;
}
return 0;
}

long double function(int n) {
    if ( n <= 0)
    return 0;
    else
    return n+function(n-1);
}