#include <bits/stdc++.h>
using namespace std;
using ll = long long;
double pi() { return 3.14; }
int sum(int a, int b) { return a + b; }
int mul(int a, int b) { return a * b; }
ll LGCD(ll x, ll y) {
  if (x % y == 0)
    return y;
  else
    return LGCD(y, x % y);
}
ll LLCM(ll x, ll y) { return x * y / LGCD(x, y); }
int GCD(int x, int y) {
  if (x % y == 0)
    return y;
  else
    return GCD(y, x % y);
}
int LCM(int x, int y) { return x * y / GCD(x, y); }
ll dpc[10000][10000];
ll combination(ll n, ll r) {
  if (n == r) {
    return dpc[n][n] = 1;
  } else if (r == 0) {
    return dpc[n][0] = 1;
  } else if (r == 1) {
    return dpc[n][1] = n;
  } else if (dpc[n][r]) {
    return dpc[n][r];
  } else {
    return dpc[n][r] = combination(n - 1, r - 1) + combination(n - 1, r);
  }
}
int main() {
  int r;
  double l;
  cin >> r;
  l = r * 2 * 3.1415926535;
  cout << l << endl;
  return 0;
}
