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

int main(){
    int r;cin >> r;
    cout << fixed << setprecision(15) <<  2*acos(-1)*r << endl;
}