#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for(long long i=0;i<n;i++)
using ll = long long;
const ll INF = (1LL<<60);

int main(){

    long double r; cin >> r;

    cout << 2*3.14159265358979*r << endl;

    return 0;
}