import java.util.Scanner;
public class Main{
    public static void main(String... args){
        Scanner sc = new Scanner(System.in);
        int u,b,h;
        u = sc.nextInt();
        b = sc.nextInt();
        h = sc.nextInt();
        System.out.printf("%.2f",(double)(u+b)*h/(double)2);
        
        
    }
}