    #include <bits/stdc++.h>
    #define rep(i,n) for(int i = 0; i < (n); i++)
     
    using namespace std; 
    using ll = long long;
    using P = pair<int,int>; 
    
    int main() {
     double r;
     cin >> r;
     
     const double pi = 3.14159265;
     	cout << 2*r*pi << '\n';
     
     return 0;
    }
    