#include <bits/stdc++.h>
#include <math.h>
using namespace std;
#define rep(i,n) for (int i=0;i<(n);i++)
typedef long long ll;
bool aaa[11111111];
const int dx[4]={1,0,-1,0};
const int dy[4]={0,1,0,-1};
ll gcd(ll a, ll b) {
  if(a < b) return gcd(b, a);
  unsigned r;
  while ((r=a%b)) {
    a = b;
    b = r;
  }
  return b;
}
 
 
int main() {
  ll a;
  float x1,x2,bx,b=50000.0001,c=5;
  double xx1,xx2,bx,aa,cc;
  aa=3.1415926535;
  cin>>xx1;
  cout<<fixed<<xx1*aa;
  //vector<vector<bool>> sinsetsu(15,vector<bool>(14)),usotsuki(15,vector<bool>(14));
  //vector<vector<int>> sss(15,vector<int>(14))
  //vector<vector<int>> sss(h,vector<int>(w));
  //vector<int> a(k),b(k);
  //char x;
}