import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, *A = map(int, read().split())

B = [A[0]]
for x in A:
    if x != B[-1]:
        B.append(x)

op = 0
if B[-1] == B[0]:
    B.pop()
    op = 1

if len(B) == len(set(B)):
    print(op)
else:
    print(-1)