import std.stdio, std.string, std.range, std.conv, std.array, std.algorithm, std.math, std.typecons;

void main() {
    auto tmp = readln.split.to!(long[]);
    const H = tmp[0], W = tmp[1];
    tmp = readln.split.to!(long[]);
    const h = tmp[0], w = tmp[1];

    writeln(H * W - (h * W + w * H - h * w));
}

