C :

#include <stdio.h>

int main(void) {
	double a, b, h;
	scanf("%lf%lf%lf", &a, &b, &h);
	printf("%.2f\n", (a+b)*h/2);
	return 0; 
}

C++ :

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

Pascal :

program ygj288;
  var
    a,b,h,d:real;
  begin
    read(a,b,h);
    d:=(a+b)*h/2;
    write(d:0:2);
  end.