#include <bits/stdc++.h>
long long sieve[1000001] = {0}, i, j;
long long m = 1e9 + 7;
long long lev = 0;
long long n, k;
long long power(long long p, long long k) {
  long long a = 1;
  while (k) {
    if (k & 1) a = (a * p) % m;
    p = (p * p) % m;
    k >>= 1;
  }
  return a;
}
using namespace std;
vector<long long> v[1000000 + 1];
long long vis[1000000 + 1] = {0};
void dfs(long long u, long long p, long long lev) {
  vis[u] = lev;
  for (auto x : v[u]) {
    if (x != p) {
      dfs(x, u, lev + 1);
    }
  }
}
bool sec(pair<int, int> &a, pair<int, int> &b) { return (a.second < b.second); }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int tc = 1;
  long long cas = 1;
  while (tc--) {
    long long r;
    cin >> r;
    cout << 3.14 * r;
    cout << "\n";
  }
}
