#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
ll mod = 1000000007;
#define _USE_MATH_DEFINES
#define print(val) print_helper(#val, val);
#define bg begin
#define pb push_back
#define pll pair<ll,ll>
#define pii pair<int32_t,int32_t>
#define loop(i,n) for(int64_t i=0; i<n; i++)
#define loop1(i,a,b) for(int64_t i=a; i<=b; i++)
#define rloop(i,n) for(int64_t i=n-1; i>=0; i--) 
#define rloop1(i,a,b) for(int64_t i=a; i>=b; i--) 
#define fast_io ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
void err() {cout << "-1\n";}
template <typename T>
void print_c(T & a) {
for_each(begin(a),end(a),[](auto & i){cout << i << " ";}); cout << "\n"; }
template <typename T, typename V>
void fill(T & collection, V val) {fill(begin(collection),end(collection),val);}
template <typename T>
void print_helper(string name, T val) {cout << name << ": " << val << "\n";}

int main() {
  fast_io
  int r; cin >> r;
  cout << setprecision(10) << M_PI*r << "\n"; 
}
