def interval(first, last):
    # Combine first and last into a list of tuples and sort them
    # Sort primarily by end point (ascending)
    # Sort secondarily by start point (descending) to handle subsets properly
    intervals = list(zip(first, last))
    intervals.sort(key=lambda x: (x[1], -x[0]))
 
    ans = 0
    p1 = -1 # Largest element in our set
    p2 = -1 # Second largest element in our set
 
    for start, end in intervals:
        # Case 1: The current interval already contains at least two points from our set
        if start <= p2:
            continue
 
        # Case 2: The current interval contains exactly one point from our set
        if start <= p1:
            # We need 1 more point. Pick the largest possible point (end).
            # If p1 is already exactly 'end', we must pick 'end - 1'.
            if p1 == end:
                p2 = end - 1
                # p1 remains 'end'
            else:
                p2 = p1
                p1 = end
            ans += 1
 
        # Case 3: The current interval contains zero points from our set
        else:
            # We need 2 points. Pick the two largest possible: 'end - 1' and 'end'
            p2 = end - 1
            p1 = end
            ans += 2
 
    return ans
 
print(interval([3,2,0,4], [6,4,2,7]))

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