#include <bits/stdc++.h>
using namespace std;
int read() {
  int x = 0, f = 1, ch = getchar();
  while (ch > '9' || ch < '0') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int main() {
  int r = read();
  printf("%.8lf\n", 2 * r * 3.1415926);
  return 0;
}
