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

T read(T)() { return readln.chomp.to!T; }
T[] reads(T)() { return readln.split.to!(T[]); }
alias readint = read!int;
alias readints = reads!int;

void main() {
    auto HW = readints;
    int H = HW[0], W = HW[1];
    auto hw = readints;
    int h = hw[0], w = hw[1];

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