#include <bits/stdc++.h>
#include <algorithm>
#include <math.h>
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
#define INF 999999999
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

using llong = long long;
using namespace std;


int main(){
    double r;
    cin >> r;
    cout << 6.28318530717958623200*r << endl;
}
