#include <iostream>
#include <numeric>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <queue>

#define rep(i, a) FOR(i, 0, a)
#define FOR(i, a, b) for(int i = a; i < b; ++i)

int main(){
	int n;
	std::cin >> n;
	printf("%.10lf\n", n * 3.5);
	return 0;
}