def longestSubstring(str1, k):
    l = 0
    max_len = 0
    counts = {}

    for r in range(len(str1)):
        # Add in an item to the window, by adding 1 to its count
        counts[str1[r]] += counts.get(str1[r], 0) + 1

        # Now we contract the window if we've seen more than
        # 'k' characters
        while len(counts) > k:
            # Take an item out of the window by subtracting
            # 1 from its ounts
            counts[str1[l]] -= 1

            # If it turns out that we've exhausted this key,
            # delete it from the dict
            if counts[str1[l]] == 0:
                del counts[str1[l]]

            # Then shift our pointer to the right
            l += 1

        max_len = max(max_len, r - l + 1)

     return max_len