#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef pair<int,int> P;
 
const int INF = 1e9;
const int mod = 1e9+7;
const double EPS = 1e-10;
const double PI = acos(-1.0);

int dp[1010];

int main()
{
    int n;
    cin >> n;
    cout << fixed << n * 3.5 << endl;
    return 0;
}