C :

#include <stdio.h>
void main()
{
	int a,b,h;
	double s;
	scanf("%d %d %d",&a,&b,&h);
	s=(a+b)*h/2.0;
	printf("%.1f\n",s);
 } 

C++ :

#include<bits/stdc++.h>
using namespace std;
int main(){

	float a,b,h;
	cin>>a>>b>>h;
	cout<<fixed<<setprecision(1)<<(a+b)*h/2<<endl;


	return 0;
}

Pascal :

var
  n:real;
  a,b,h:longint;
begin
  read(a,b,h);
  n:=(a+b)*h/2;
  write(n:0:1);
end.

Java :

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int h = sc.nextInt();
		double s ;
		s = ((a + b)*h )/(2*1.0);
		System.out.printf("%.1f",s);
	}
}

Python :

a,b,h=map(int,input().split())
s=(a+b)*h/2
print('%.1f'% s)