#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
#define puts(i) cout<<i<<endl
#define div 1e9+7
#define pi 3.14159265358979
using ll = long long;
using P = pair<int,int>;
int main(){
	int r;
	cin>>r;
	cout << setprecision(20)<<2*r*pi<<endl;
}
