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) {
        /* Enter your code here. Read input from STDIN. Print output to STDOUT. Your class should be named Solution. */
        Scanner in = new Scanner (System.in);
        int a= in.nextInt(), b = in.nextInt(), t = in.nextInt();
        while (in.hasNext())
            {
                System.out.println(in.nextInt());
        }
        int result;
        result = (int) ( 0.5 * a + 0.5 * b * t);
        System.out.println(result);
        
    }
}