#include <cstdio>
#include <cmath>
using namespace std;

int main(){
	double a, l, x;
	while( scanf("%lf %lf %lf", &a, &l, &x) != EOF ){
		double ABC = 0.5 * a * sqrt( l * l - (a/2.0) * (a/2.0));
		double ADC = 0.5 * l * sqrt( ((l+x)/2.0) * ((l+x)/2.0) - (l/2.0) * (l/2.0) );
		double ans = ABC + 2.0 * ADC;
		printf("%.10f\n", ans);
	}
}