5.2.10

#include <iostream>
#include <iomanip>
#include <algorithm>
#include <cmath>

using namespace std;

int main()
{
    int n, k;
    double x, s, d;
    s = 0;
    cin >> k >> x;
    d = ((-1) * x / 2);
    for (n = 1; n <= k; n++) {
        s += d / (7 * n);
        d *= ((-1) * x / 2);
    }
    cout << s;
    return 0;
}