class Problem0799:
    def solve(this):
        a,b,c,d = list(map(int, input().split()))
        
        res = 0
        for i in range(a,b+1):
            for j in range(c,d+1):
                if i != j:
                    res += 1
        print(res)

if __name__ == "__main__":
    problem = Problem0799()
    problem.solve()