#include <bits/stdc++.h>
#define rep(i,n) for(int i=0; i<(n); ++i)
using namespace std;
const double PI=3.14159265358979323846;

const string YES = "Yes";
const string NO = "No";

void solve(long long N){
    double result = (double)N*2*PI;
    printf("%8.20f\n", result);
}

int main(){
    long long N;
    scanf("%lld",&N);
    solve(N);
    return 0;
}
