import std.functional,
       std.algorithm,
       std.container,
       std.typetuple,
       std.typecons,
       std.bigint,
       std.string,
       std.traits,
       std.array,
       std.range,
       std.stdio,
       std.conv;

void main() {
  int[] HW = readln.chomp.split.to!(int[]);
  int H = HW[0],
      W = HW[1];
  int[] hw = readln.chomp.split.to!(int[]);
  int h = hw[0],
      w = hw[1];

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