#include <iostream>
    #include <cmath>
    using namespace std;
     
    int main() {
    	double E, a = 2, sum = 2;
    	cin >> E;
    	for(int i=1; abs(a*=2.0/(i+1))>=E;i++){
    		sum+=a;
    	}
    	cout << sum;
    	return 0;
    }