using namespace std;
#include <bits/stdc++.h>
#define BEGIN ios_base::sync_with_stdio(0);cin.tie(0);
#define END return EXIT_SUCCESS;
#define FOR(I,A,B) for(int (I)=(A);(I)<(B);++(I))
#define ALL(C) (C).begin(),(C).end()
inline void solve()
{
	int N;
	cin >> N;
	cout << 3.5*N << endl;
}
int main(int argc,char**argv)
{
	BEGIN
	solve();
	END
}