a,b,c,d=map(int,input().split())
if(a>c):
    a,b,c,d=c,d,a,b
    pass;
if(b<c):
    print((b-a+1)*(d-c+1))
    pass;
else:
    print((b-a+1)*(d-c+1)-(b-c+1))