import std.stdio, std.range, std.conv, std.string;
import std.algorithm.comparison, std.algorithm.iteration, std.algorithm.mutation, std.algorithm.searching, std.algorithm.setops, std.algorithm.sorting;

void main()
{
    int[] input = readln().split.to!(int[]);
    int H = input[0];
    int W = input[1];
    input = readln().split.to!(int[]);
    int h = input[0];
    int w = input[1];
    writeln((H-h)*(W-w));
}