to fille

test.py

import sys

lines = []
for line in sys.stdin:
    lines.append(line)

numbers = map(int, lines[1].split())

max = numbers[-1]
min = numbers[0]
delta = (max-min) / (len(numbers))

for i in range(1, len(numbers)):
    expected = numbers[i-1] + delta

    if numbers[i] != expected:
        print expected
        break