def main():
    i = input()
    lst = i.split(' ')
    A,B,C,D=int(lst[0]),int(lst[1]),int(lst[2]),int(lst[3])
    a = set([i for i in range(A,B+1)])
    b = set([i for i in range(C,D+1)])
    c = len(a)*len(b)
    print(c-len(a&b))


if __name__ == '__main__':
    main()