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

typedef long long ll;

const int M = (int)1e2;
const int inf = 0x3f3f3f3f;
const ll mod = (ll)1e9 + 7;
const double eps = 1e-8;

int main()
{
    int r; scanf("%d", &r);
    printf("%.12f\n", acos(-1.0) * r);
    return 0;
}
