C :

#include<stdio.h>
int main()
{
	double a, b, h;
	double area;
	scanf("%lf%lf%lf", &a, &b, &h);
	if (a>0 && b>0 && h>0)
	{
		area = (1.0)*(((a + b)*h) / 2);
		printf("%.2lf\n", area);
	}
	else
		return 0;
}

C++ :

#include<cstdio>
using namespace std;
int main()
{
    float a,b,h,s;
    scanf("%f%f%f",&a,&b,&h);
    s=(a+b)*h/2;
    printf("%.2f",s);
    return 0;
}

Pascal :

var
  a,b,h,s:real;
begin
  read(a,b,h);
  s:=(a+b)*h/2;
 write(s:0:2);
end.