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

auto keta(ll n) {
    ll res;
    for(ll i = 0; i < 100; i++) {
        n = n / 10;

        if(n == 0) {
            res = i + 1;
            break;
        }
    }
    return res;
}

int main() {
    double b;
    cin>>b;
    cout<<b*3.141<<endl;
}
