#include <bits/stdc++.h>
#define int long long
#define f first
#define s second
using namespace std;
int32_t main(){
    if(fopen("input.txt", "r"))
    freopen("input.txt", "r", stdin),
    freopen("output.txt", "w", stdout);
    ios::sync_with_stdio(0),cin.tie(0);
    long double r;
    cin >> r;
    long double pi=M_PI;
    cout << setprecision(20) << 2*pi*r ;
}