#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
int read() {
  char x = getchar();
  int u = 0, fg = 0;
  while (!isdigit(x)) {
    if (x == '-') fg = 1;
    x = getchar();
  }
  while (isdigit(x)) {
    u = (u << 3) + (u << 1) + (x ^ 48);
    x = getchar();
  }
  return fg ? -u : u;
}
int main() {
  int r;
  scanf("%d", &r);
  printf("%.18lf\n", r * pi * 2);
  return 0;
}
