#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

#define INF 1000000000
#define EPS 1e-9
#define PI acos(-1)

typedef long long ll;
typedef pair<int, int> P;

int main(){

int n;
cin >> n;
cout << (double)3.5*(double)n << endl;
return 0;
}