#include <iostream>
#include <vector>
#include <algorithm>
#include <iomanip>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)

using namespace std;

typedef long long int lli;

int main() {
    int N;
    cin >> N;
    /*
    vector<vector<lli>> dp(N + 1, vector<lli>(6 * N + 1, 0));
    dp[0][0] = 1;
    REP (i, 0, N) REP (j, 0, 6 * N + 1) {
        REP (k, 1, 7) {
            if (j + k <= 6 * N) dp[i + 1][j + k] += dp[i][j];
        }
    }
    double ans = 0.0;
    REP (i, 0, 6 * N + 1) ans += dp[N][i] * i;
    REP (i, 0, N) ans /= 6;
    cout << fixed << setprecision(10) << ans << endl;
    */
    cout << fixed << setprecision(10) << 3.5 * N << endl;
    return 0;
}