#include<bits/stdc++.h>
using namespace std;
#define ll long long 
#define rep(i,n) for(int i=0;i<n;i++)
#define INF 999999999
const int mod = 1000000007;
ll cnt = 0;

int main(){
  int n;
  cin >> n;
  cout << M_PI*n << endl;
}