    #include <bits/stdc++.h>
    using namespace std;
    #define COUT(x) cout<<(x)<<endl
    typedef long long ll;
    template<class T> inline void chmax(T& a, T b) {
      if (a < b) a = b;
    }
     
    template<class T> inline void chmin(T& a, T b) {
      if (a > b) a = b;
    }
     
    const long long INF = 1LL << 60;
    int main() {
      int r;
      cin >> r;
      const double pi = 3.1415926535;
      double leng = 2 * pi * r;
      cout << leng << endl;
      return 0;
     
    }