C :

#include"stdio.h"
#include"math.h"
int main()
{
	//freopen("in.txt","r",stdin);
	//freopen("out.txt","r+",stdout);
	double s=0,a=0,b=0,h=0;
	while(~scanf("%lf%lf%lf",&a,&b,&h))
	{
		s=4*a*b+2*b*sqrt(b*b/4+(h-a)*(h-a));
		printf("%.3lf\n",s);
	}
	return 0;
}

C++ :

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

int main() {
    double a, b, h;
    while(cin>>a>>b>>h) {
        h -= a;
        double s = a * b * 4 + 2 * b * sqrt(h*h + b*b/4.0);
        printf("%.3lf\n", s);
    }
    return 0;
}