#include <bits/stdc++.h>

#define rep(i, N) for (ll i = 0; i < N; i++)
#define rep1(i, N) for (ll i = 1; i <= N; i++)
#define repr(i, N) for (ll i = N-1; i >= 0; i--)
#define repr1(i, N) for (ll i = N; i > 0; i--)

#define INF 1000000007

using ll = long long;
using namespace std;

typedef pair<int,int> P;

int main(){
    int N;
    cin >> N;

    cout << 2＊N*M_PI << endl;
    //cout << endl;
    return 0;
}
