import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner scn = new Scanner(System.in);
        double a, b, t;
        a = scn.nextDouble();
            b = scn.nextDouble();
            t = scn.nextDouble();
        double value = 0.5*a*t + 0.5*b*t;
        System.out.print((int)value);
    }
}