cs=Array.new(101,0)
a,b,c,d=gets.split(" ").map{|e| e.to_i}
a.upto(b){|e| cs[e]+=1}
c.upto(d){|e| cs[e]+=1}
puts (b-a+1)*(d-c+1)-cs.count{|e| e==2}