#include <bits/stdc++.h>
using namespace std;

#define fast() {ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);}
#define o(a) { cout << #a << ":" << (a) <<" ";}
#define ov(a) { cout << "["; \
	for(int i = 0; i < (a).size(); ++i){ \
		cout << i <<":" << a[i] <<" "; \
		if(i % 10 == 9) cout <<"\n"; \
		} \
	cout << "]"; \
}
#define ovv(a) { cout << "["; \
	for(int i = 0; i < a.size(); ++i){ \
		for(int j = 0; j < a[i].size(); ++j){ \
			cout << "(" << i <<"," <<j <<")" <<":" << a[i][j] <<" "; \
		} \
		cout << "\n"; \
	} \
	cout << "]"; \
}
#define op(a) { cout <<"[" << #a << ".fi:"<<a.first; \
					cout << " "<< #a << ".se:" << a.second <<"]"; }
//priority_queue <int>a;
#define ll long long
const int mod = 1e9 + 7;
const int nax = 1e5 + 5;
int n, m, k, q;

int main(){
	fast();
	int circle_radius;
   float PI_VALUE=3.14159265359, circle_area, circle_circumf;

   scanf("%d",&circle_radius);

   //Calculate and display Area
   circle_area = PI_VALUE * circle_radius * circle_radius;

   //Caluclate and display Circumference
   circle_circumf = 2 * PI_VALUE * circle_radius;
   printf("%.10f",circle_circumf);

   return(0);

}
