import sequtils,strutils
var
    N = stdin.readline.parseInt
    A = stdin.readline.split.map(parseInt)
    used : array[100010, bool]
    visited : array[100010,bool]
    ans = 0

proc dfs(p : int) =
    visited[p] = true
    var np = (p + 1) mod N

    if A[np] == A[p] and visited[np] == false:
        dfs(np)

    np = p - 1
    if np == -1 and A[p] == A[A.high] and visited[A.high] == false:
        ans = 1
        dfs(A.high)

    elif np >= 0 and A[np] == A[p] and visited[np] == false:
        dfs(np)

for i,a in A:
    if visited[i]:
        continue
    if used[a]:
        ans = -1
        break
    used[a] = true
    dfs(i)

echo ans