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

public class Solution {

    static long countArray(int n, int k, int x)
    {
        long sum=0;
        long m=n-2;
        long nume=k*m;
        sum=nume/2;
        return sum;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int k = in.nextInt();
        int x = in.nextInt();
        long answer = countArray(n, k, x);
        System.out.println(answer);
        in.close();
    }
}