Write a program to count the number of days between two dates.

The two dates are given as strings, their format is `YYYY-MM-DD` as shown in the examples.

**Example 1:**

**Input:** date1 = "2019-06-29", date2 = "2019-06-30"
**Output:** 1

**Example 2:**

**Input:** date1 = "2020-01-15", date2 = "2019-12-31"
**Output:** 15

**Constraints:**

*   The given dates are valid dates between the years `1971` and `2100`.


def maxLength(arr):
    dp = ['']
    maxLen = 0

    for s in arr:
        if len(set(s)) < len(s):
            continue
        for i in range(len(dp) - 1, -1, -1):
            tmp = dp[i] + s
            if len(set(tmp)) == len(tmp):
                dp.append(tmp)
                maxLen = max(maxLen, len(tmp))

    return maxLen
