import sys, re from fractions import Fraction from math import ceil, floor, sqrt, pi, factorial, gcd from copy import deepcopy from collections import Counter, deque, defaultdict from heapq import heapify, heappop, heappush from itertools import accumulate, product, combinations, combinations_with_replacement, permutations from bisect import bisect, bisect_left, bisect_right from functools import reduce from decimal import Decimal, getcontext, ROUND_HALF_UP def i_input(): return int(input()) def i_map(): return map(int, input().split()) def i_list(): return list(i_map()) def i_row(N): return [i_input() for _ in range(N)] def i_row_list(N): return [i_list() for _ in range(N)] def s_input(): return input() def s_map(): return input().split() def s_list(): return list(s_map()) def s_row(N): return [s_input for _ in range(N)] def s_row_str(N): return [s_list() for _ in range(N)] def s_row_list(N): return [list(s_input()) for _ in range(N)] def lcm(a, b): return a * b // gcd(a, b) def get_distance(x1, y1, x2, y2): d = sqrt((x2 - x1) ** 2 + (y2 - y1) ** 2) return d def rotate(table): n_fild = [] for x in zip(*table[::-1]): n_fild.append(x) return n_fild sys.setrecursionlimit(10 ** 7) INF = float('inf') MOD = 10 ** 9 + 7 MOD2 = 998244353 def main(): A, B, C, D = i_map() ans = 0 for i in range(A, B+1): for j in range(C, D+1): if i != j: ans += 1 print(ans) if __name__ == '__main__': main()