#include <bits/stdc++.h>
using namespace std;
long long int modp(long long int a, long long int p, long long int n) {
  int d[100];
  int i, j, k, l;
  if (n == 0) return 1;
  for (i = 0; n >= 1; i++) {
    d[i] = n % 2;
    n /= 2;
  }
  l = i;
  long long int e = 1;
  if (d[0] == 1) e *= (a % p);
  for (i = 1; i < l; i++) {
    a *= a;
    a = a % p;
    if (d[i] == 1) {
      e *= a;
      e = e % p;
    }
  }
  return e % p;
}
long long int modInverse(long long int n, long long int p) {
  return modp(n, p, p - 2);
}
long long int nCrModPFermat(long long int n, long long int r, long long int p) {
  if (r == 0) return 1;
  long long int fac[n + 1];
  fac[0] = 1;
  for (long long int i = 1; i <= n; i++) fac[i] = fac[i - 1] * i % p;
  return (fac[n] * modInverse(fac[r], p) % p * modInverse(fac[n - r], p) % p) %
         p;
}
long long int gcd(long long int b, long long int a) {
  if (a < b) return gcd(a, b);
  if (a % b == 0) return b;
  return gcd(a % b, b);
}
long long int p1 = 1000000007, p2 = 998244353;
long long int l, r, mid, ans;
long long int n, m, i, j, k;
long long int x, y, n1, a, b, n2, h, z, c;
queue<long long int> q;
long long int d, p = 1000000000;
vector<vector<long long int> > v(100001);
void bfs(long long int x, vector<long long int>& d) {
  d[x] = 0;
  q.push(x);
  while (!q.empty()) {
    x = q.front();
    q.pop();
    l = v[x].size();
    for (i = 0; i < l; i++) {
      y = v[x][i];
      if (d[y] == (long long int)1e18) {
        d[y] = d[x] + 1;
        q.push(y);
      }
    }
  }
}
void solve() {
  cin >> n;
  float p = 3.14159;
  cout << 2 * p * n;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t = 1;
  for (int i = 0; i < t; i++) {
    solve();
    cout << "\n";
  }
  return 0;
}
