#include<bits/stdc++.h>
#define rep(i,n)for(int i=0;i<n;i++)
using namespace std;

int main() {
	double a, l, x;
	while (~scanf("%lf%lf%lf", &a, &l, &x)) {
		double s1 = (a + l + l) / 2, s2 = (l + l + x) / 2;
		double d = (l + x) / 2;
		printf("%.10lf\n", sqrt(s1*(s1 - a)*(s1 - l)*(s1 - l)) + sqrt(s2*(s2 - d)*(s2 - d)*(s2 - l)) * 2);
	}
}