#include <bits/stdc++.h>
using namespace std;
inline int read() {
  register int ans = 0;
  register bool f = 1;
  register char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = 0;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    ans = (ans << 3) + (ans << 1) + (ch ^ 48);
    ch = getchar();
  }
  return f ? ans : ~(ans - 1);
}
signed main(void) {
  double r = read();
  r *= 2;
  double pai = 3.14159265358;
  printf("%lf\n", pai * r);
  return 0;
}
