import java.io.*;
import java.util.*;

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int factorA = in.nextInt();
        int factorB = in.nextInt();
        int tiempo = in.nextInt();
              
        double solucion = 0;
        
        solucion = 0.5 * factorA * tiempo;
        solucion +=  0.5 * factorB * tiempo;
        
        int sol = (int) solucion;
        System.out.println(sol);
    }
}