
#include<bits/stdc++.h>
using namespace std;
#define gearup ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);

#define pb push_back
#define F first
#define S second

#define all(v) v.begin(),v.end()
#define allr(v) v.rbegin(),v.rend()
#define int long long
#define ll long long
const int M=1e9+7;

int dx[]={1,-1,0,0,1,1,-1,-1};
int dy[]={0,0,1,-1,1,-1,1,-1};

int expo(int x, int y) {int res=1;x=x%M;while(y>0){if(y&1)res=(1ll*res*x)%M;
	y=y>>1;x=(1ll*x*x)%M;} return res;}
	

signed main(){
	gearup;
	int t = 1;
	//cin >> t;
	while(t--){
		int r;
		cin >> r;
		long double ans = 2 * 3.141592 * r ;
		cout << ans;
	}
	return 0;
}