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) {
        DecimalFormat format = new DecimalFormat();
        format.setDecimalSeparatorAlwaysShown(false);
        
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int t = in.nextInt();
        
        double res = (0.5*a*t) + (0.5*b*t);
        
        System.out.println(format.format(res));
    }
}