#include <bits/stdc++.h>
using namespace std;
#define rep(i,n); for(long long i = 0;i < (n);i++)
using ll = long long;
using vl = vector<long long>;
using P = pair<long long,long long>;
 
int main(){
    double r; cin >> r;
    cout << 2*r*3.1415 << endl;
}