# python template for atcoder1
import sys
sys.setrecursionlimit(10**9)
input = sys.stdin.readline

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

aka = set(range(A, B+1))
kuro = set(range(C, D+1))

common = len(aka & kuro)

ans = (B-A+1)*(D-C+1)-common
print(ans)