N = int(input())
A = list(map(int, input().split()))
B = []
pre = -1
for a in A:
    if pre != a:
        B.append(a)
    pre = a

L = len(B)
X = len(set(B))
# 長さLに対して文字種が同じなら0
if X == L:
    print(0)
elif X == L - 1 and B[0] == B[-1]:
    print(1)
else:
    print(-1)