a,b,c,d = map(int,input().split())
ans = (b-a+1) * (d-c+1)
if a >= c:
    a,c =c,a
    b,d = d,b

if c <= b and d <= b:
    ans -= d-c+1
elif c <= b:
    ans -= b-c+1
else:
    pass

print(ans)