C :

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

C++ :

#include<cstdio>
#include<iostream>
using namespace std;
int main()
{
	float a,b,h,s;
	cin>>a>>b>>h;
	s=(a+b)*h/2;
	printf("%0.2f\n",s);
	return 0;
}

Pascal :

var
   a,b,c:extended;
begin
   readln(a,b,c);
   writeln(((a+b)*c)/2:0:2);
end.

Python :

a,b,c = raw_input().split()
a = float(a)
b = float(b)
c = float(c)
print "%.2f" % (((a + b) * c) / 2.0)