#include <bits/stdc++.h>
#define int long long
#define sz(x) (int)(x.size())
using namespace std;

const int N = 2e5 + 5, inf = 1e18, mod = 1e9 + 7;


int32_t main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);  


    int r;
    cin >> r;
    cout << fixed << setprecision(12);
    double ans = 2 * (3.14159 * r);
    cout << ans;
}