arr1 = $stdin.gets.chomp.split(" ")
arr1.map! do |a_j|
 a_j.to_i
end
arr2 = $stdin.gets.chomp.split(" ")
arr2.map! do |a_j|
 a_j.to_i
end

x = arr1[0]*arr1[1] - (arr2[0]*arr1[1]+arr2[1]*arr1[0]-arr2[0]*arr2[1])
puts x