#include <bits/stdc++.h>
using namespace std;
 
#define FOR(i, l, r) for(size_t i = (l); i < (r); i = i + 1)
#define REP(i, n) FOR(i, 0, n)
const double PI=3.141592;

using ll = long long;
 
int main() {
    int R;
    cin >> R;
    cout << 2 * PI * R<< endl;
 
    return 0;
}