#include<bits/stdc++.h>
#define endl '\n'
#define pb push_back
#define fr first
#define sc second
#define ll long long int
#define bit(idx) idx&(-idx)
#define bin(x, a) bitset<a>(x)
#define all(A) A.begin(), A.end()
#define debug(x) cout << #x << " = " << x << endl;
using namespace std;

const long double PI = 3.14159;

int main(){
	ios_base::sync_with_stdio(NULL); cin.tie(); cout.tie();
	long double r; cin >> r;
	cout << (long double) 2* r * PI << endl;
}
/**

*/