#include<bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < n; i++)
using namespace std;
using ll = long long;

int main(){

    double n,ans=0;
    cin >> n;
    ans = n * 2 * 3.1415;
    cout << std::fixed << std::setprecision(8) << ans << endl;
	return 0;
}
