#include <iostream>
#include <iomanip>
using namespace std;

int main() {
	double t1,t2,t3;
        cin >> t1 >> t2 >> t3;
        double t = 1/(1/t1+1/t2+1/t3) ;
        if( (1/t1+1/t2+1/t3)!=0) cout << setprecision(2) << fixed << t;
	return 0;
}