import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String S = sc.next();
        String[] strings = S.split(",");
        int u = Integer.parseInt(strings[0]);
        int b = Integer.parseInt(strings[1]);
        int h = Integer.parseInt(strings[2]);
        double a = (double) ((u + b) * h) / 2;
        System.out.printf("%s", String.format("%.2f", a));
    }
}