#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define INF (1LL<<50)
#define MOD 1000000007

int main(){
    double R;
    cin >> R;
    cout << 2*R*3.1415 << endl;
    return 0;
}