from sys import stdin

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

    black = D - C + 1
    ans = 0
    for i in range(A, B + 1):
        if C <= i <= D:
            ans += black - 1
        else:
            ans += black
    print(ans)

input = lambda: stdin.readline()
main()