#include <bits/stdc++.h>
using namespace std;
#define vi vector<int>
#define pi pair<int,int>
#define ll long long
#define rep(i,a,b) for(int i=int(a); i<=int(b);i++)
#define repv(vec,v) for(v=(vec).begin(); v!=(vec).end();v++)
#define sz(vec) (int)(vec).size()
#define PB push_back
#define F first
#define S second

void solve(){
	
}


int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	double r; cin >> r;
	cout << 2*r*M_PI;

	
	return 0;
}
