#include <bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < n; ++i)
typedef long long ll;
using namespace std;
const int INF = 1e9;

int main() {
    int n;
    cin >> n;

    if (n % 2 == 0)
        cout << n / 2 * 7 << endl;

    else
        printf("%.1f\n", n / 2 * 7 + 3.5);

    return 0;
}