#(B 竏・A + 1) ( D 竏・C + 1) 竏・m a x ( 0 , m i n ( B , D) 竏・m a x ( A , C) + 1
a,b,c,d=map(int,input().split())
print((b-a+1)*(d-c+1)-max(0,min(b,d)-max(a,c)+1))