#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;
}
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