#include <bits/stdc++.h>
using namespace std;
const int N = 19;
inline int read() {
  int x = 0, ff = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') ff = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  return x * ff;
}
int main() {
  double pai = 3.1415926535;
  int n = read();
  printf("%.20lf", n * pai);
  return 0;
}
