#include<stdio.h>
#include<algorithm>
#include<math.h>
using namespace std;
double calc(double a,double b,double c){
	double s=(a+b+c)/2;
	return sqrt(s*(s-a)*(s-b)*(s-c));
}
int main(){
	int a,b,c;
	while(~scanf("%d%d%d",&a,&b,&c)){
		printf("%.12f\n",calc(a,b,b)+calc(b,0.5*(b+c),0.5*(b+c))*2);
	}
}