#include <bits/stdc++.h>
int compare_int(const void *a, const void *b) { return *(int *)a - *(int *)b; }
long long factorial(long long n, long long depth) {
  if (n > 1) {
    depth++;
    return factorial(n / 2, depth);
  } else {
    return depth;
  }
}
int min(int a, int b) {
  if (a > b)
    return b;
  else
    return a;
}
int distance(int a, int b) { return abs(a - b) * abs(a - b); }
int main() {
  double a, b, c, d, e, X, Y;
  int A[200003] = {};
  int B[100003] = {};
  int C[100003] = {};
  int flg = 0;
  char s[100001] = {};
  char t[100001] = {};
  int i, j, k;
  long long p = 0, q = 0;
  int sosuu[46] = {2,   3,   5,   7,   11,  13,  17,  19,  23,  29,  31,  37,
                   41,  43,  47,  53,  59,  61,  67,  71,  73,  79,  83,  89,
                   97,  101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151,
                   157, 163, 167, 173, 179, 181, 191, 193, 197, 199};
  int min = 100000001;
  int len = 0;
  int l = 0;
  int cnt = 0;
  int num = 0;
  int num1 = 0;
  int num2 = 0;
  long long ans = 0;
  int ans_ac = 0;
  int ans_wa = 0;
  int max = 0;
  long long sum = 0;
  int addr = 0;
  int ptn = 0;
  int tmp = 0;
  double per = 0;
  scanf("%llf", &a);
  printf("%llf\n", 3.14 * 2 * a);
  return 0;
}
