#include <bits/stdc++.h>
#define MOD (long long)(1E9+7)
#define rep(i, n) for(int i = 0; i < n; i++)

using namespace std;


long long extgcd(long long a)
{
	long long b = MOD;
	long long u = 1, v = 0, q; 
	while ( b > 0 ) {
		q = a / b; 
		a -= q * b; swap(a,b);
		u -= q * v; swap(u,v);
	}
	u %= MOD;
	while (u<0)
		u+=MOD;
	return u;
}
long long comb(long long n,long long k)
{
	long long t = 1;
	long long j = k;
	for(long long i = n; i  >= n-k+1; i--) {
		t *= i;
		t %= MOD;
		t *= extgcd(j);
		t %= MOD;
		j--;
	}  
  	while (t<0)
		t += MOD;
	return t;
}

int main(){
	int n;
	cin >>n;
	prinft("%8.8lf\n", 3.14159*(long long)n);
}