#include<cmath>
#include<cstdio>

using namespace std;

double area(double a,double b,double c){
	return sqrt((a+b+c)*(b+c-a)*(c+a-b)*(a+b-c))/4;
}

int main(){
	for(int a,l,x;~scanf("%d%d%d",&a,&l,&x);) printf("%.9f\n",area(a,l,l)+2*area(l,(l+x)/2.,(l+x)/2.));
	return 0;
}