#include <bits/stdc++.h>
using namespace std;
#define int long long
#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define SORT(x) sort(x.begin(), x.end())
#define RS(x) rsort(x.begin(),x,end()))
#define REVE(x) reverse(x.begin(), x.end())
#define ALL(x) (x).begin(), (x).end()
typedef long double ld;
signed main(){
double R;
 cin >> R;
 cout << 2*R*M_PI; 
}
