#include<bits/stdc++.h>
using namespace std;
double r, pi = 3.1415926, c;
int read()
{
  int x = 0, w = 1;
  char ch = 0;
  while (ch < '0' || ch > '9') {
    if (ch == '-') w = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + (ch - '0');
    ch = getchar();
  }
  return x * w;
}
inline void write(int x)
{
  static int sta[35];
  int top = 0;
  do {
    sta[top++] = x % 10, x /= 10;
  } while (x);
  while (top) putchar(sta[-- top] + 48);
}
int main()
{
    r = read();
    c = 2 * r * pi;
    printf("%.3f", c);
    return 0;
}