from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline

A,B,C,D = map(int,input().split())

ans = 0
for i in range(A,B+1):
    for j in range(C,D+1):
        if i!=j:
            ans += 1
print(ans)