python判断等差数列

import sys
n = int(sys.stdin.readline().strip())
s = sys.stdin.readline()
s = list(map(int, s.split(' ')))
print(n)
print(s)
for i in range(len(s)-1):
        for j in range(i+1, len(s)):
            if s[i] >= s[j]:
                s[i], s[j] = s[j], s[i]
for j in range(1, len(s)-1):
    if s[j] - s[j-1] == s[j+1] - s[j]:
        flag = 1
    else:
        flag = 0
if flag == 1:
    print('Possible')
else:
    print('Impossible')