import sys

sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.buffer.readline())
def LI(): return list(map(int, sys.stdin.buffer.readline().split()))
def LI1(): return list(map(int1, sys.stdin.buffer.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def BI(): return sys.stdin.buffer.readline().rstrip()
def SI(): return sys.stdin.buffer.readline().rstrip().decode()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
dij = [(0, 1), (1, 0), (1, 1), (1, -1)]
inf = 10**16
# md = 998244353
md = 10**9+7

def RLE(s):
    kk = []
    vv = []
    for c in s:
        if kk and kk[-1] == c:
            vv[-1] += 1
        else:
            kk.append(c)
            vv.append(1)
    return kk, vv

n = II()
aa = LI()
kk, vv = RLE(aa)
if len(set(kk)) == len(kk):
    print(0)
elif kk[0]==kk[-1] and len(set(kk))==len(kk)-1:
    print(1)
else:
    print(-1)