# -*- coding: utf-8 -*-

a,b,c,d = map(int,input().split())

ans = (b-a+1)*(d-c+1)

if d < a or b < c:
    pass
elif a < c and d < b:
    ans -= d-c+1
elif c < a and b < d:
    ans -= b-a+1
else:
    ans -= min(b-c+1,d-a+1)

print(ans)