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

int main()
{
	double a, l, x;
	
	while (scanf("%lf %lf %lf", &a, &l, &x) != EOF){
		double A = a * sqrt(l * l - a * a / 4) / 2;
		double B = l * sqrt((l + x) * (l + x) / 4 - l * l / 4) / 2;
		
		printf("%.10lf\n", A + B + B);
	}
	
	return 0;
}