#include <bits/stdc++.h>
#define pi acos(-1)
using namespace std;

const int N = 2e5 + 10, mod = 1e9 + 7;



int main() {
    // ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int tc = 1; 
    //scanf("%d", &tc);
    while(tc--) {
        int r; cin >> r;
        printf("%.20lf\n", 2 * pi * r);
    }    
    return 0;
}