//##MohK..
//21/04/2020....6.00...
//Ya Rab....
#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=long double;
int main(){
    ld a;
    cin>>a;
    cout<<fixed<<setprecision(2)<<2*a*3.141;

    return 0;
}
