#include <bits/stdc++.h>
using namespace std;

typedef long long LL;

#define REP(i, n) for(int i = 0; i < (int)(n); i++)
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define ALL(x) (x).begin(),(x).end()
const int IINF = 1e9;
const LL LINF = 1e18;
const LL MOD = 1e9+7;

int main() {
    double R;
    cin >> R;
    printf("%.5f\n", 2*R*M_PI);
    return 0;
}
