def pair_sum(values, target):
    left, right = 0, len(values) - 1
    while left < right:
        total = values[left] + values[right]
        if total == target:
            return values[left], values[right]
        if total < target:
            left += 1
        else:
            right -= 1
    return None

print(pair_sum([-2, 3, 5, 9], 8))
print(pair_sum([2], 4))