#include <bits/stdc++.h>

using namespace std;

#define ll long long 
#define pi 3.14159265358979323

void solve(){
    double r;
    cin>>r;
    cout<<fixed<<setprecision(10)<<pi*2*r;
}

int main() {
    int t = 1;
    while(t--)
        solve();
    return 0;
}

