import std.conv, std.stdio;
import std.algorithm, std.array, std.range, std.string;
void main()
{
    auto
        buf0 = readln.chomp.split.to!(int[]),
        buf1 = readln.chomp.split.to!(int[]);
    ((buf0[0] - buf1[0]) * (buf0[1] - buf1[1])).writeln;
}
