#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vec;
typedef vector<string> vecstr;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define NO printf("NO")
#define YES printf("YES")

int main() {
    ll n;
    cin >> n;
    cout << 3.5*n << endl;
}