One day, Twilight Sparkle is interested in how to sort a sequence of integers a1, a2, ..., an in non-decreasing order. Being a young unicorn, the only operation she can perform is a unit shift. That is, she can move the last element of the sequence to its beginning:

a1, a2, ..., an → an, a1, a2, ..., an - 1.

Help Twilight Sparkle to calculate: what is the minimum number of operations that she needs to sort the sequence?

Input

The first line contains an integer n (2 ≤ n ≤ 105). The second line contains n integer numbers a1, a2, ..., an (1 ≤ ai ≤ 105).

Output

If it's impossible to sort the sequence output -1. Otherwise output the minimum number of operations Twilight Sparkle needs to sort it.

Examples

Input

2
2 1


Output

1


Input

3
1 3 2


Output

-1


Input

2
1 2


Output

0
"""
    Author: Sagar Pandey

"""
# ---------------------------------------------------Import Libraries---------------------------------------------------
import sys
import time
import os
from math import sqrt, log, log2, ceil, log10, gcd, floor, pow, sin, cos, tan, pi, inf, factorial
from copy import copy, deepcopy
from sys import exit, stdin, stdout
from collections import Counter, defaultdict, deque
from itertools import permutations
import heapq
from bisect import bisect_left as bl
# If the element is already present in the list,
# the left most position where element has to be inserted is returned.
from bisect import bisect_right as br
from bisect import bisect

# If the element is already present in the list,
# the right most position where element has to be inserted is r

# ---------------------------------------------------Global Variables---------------------------------------------------
# sys.setrecursionlimit(100000000)
mod = 1000000007
# ---------------------------------------------------Helper Functions---------------------------------------------------
iinp = lambda: int(sys.stdin.readline())
inp = lambda: sys.stdin.readline().strip()
strl = lambda: list(inp().strip().split(" "))
intl = lambda: list(map(int, inp().split(" ")))
mint = lambda: map(int, inp().split())
flol = lambda: list(map(float, inp().split(" ")))
flush = lambda: stdout.flush()


def permute(nums):
    def fun(arr, nums, cur, v):
        if len(cur) == len(nums):
            arr.append(cur.copy())
        i = 0
        while i < len(nums):
            if v[i]:
                i += 1
                continue
            else:
                cur.append(nums[i])
                v[i] = 1
                fun(arr, nums, cur, v)
                cur.pop()
                v[i] = 0
                i += 1
            # while i<len(nums) and nums[i]==nums[i-1]:i+=1    # Uncomment for unique permutations
        return arr

    res = []
    nums.sort()
    v = [0] * len(nums)
    return fun(res, nums, [], v)


def subsets(res, index, arr, cur):
    res.append(cur.copy())
    for i in range(index, len(arr)):
        cur.append(arr[i])
        subsets(res, i + 1, arr, cur)
        cur.pop()
    return res


def sieve(N):
    root = int(sqrt(N))
    primes = [1] * (N + 1)
    primes[0], primes[1] = 0, 0
    for i in range(2, root + 1):
        if primes[i]:
            for j in range(i * i, N + 1, i):
                primes[j] = 0
    return primes


def bs(arr, l, r, x):
    if x < arr[0] or x > arr[len(arr) - 1]:
        return -1
    while l <= r:
        mid = l + (r - l) // 2
        if arr[mid] == x:
            return mid
        elif arr[mid] < x:
            l = mid + 1
        else:
            r = mid - 1
    return -1


def isPrime(n):
    if n <= 1: return False
    if n <= 3: return True
    if n % 2 == 0 or n % 3 == 0: return False
    p = int(sqrt(n))
    for i in range(5, p + 1, 6):
        if n % i == 0 or n % (i + 2) == 0:
            return False
    return True


# -------------------------------------------------------Functions------------------------------------------------------
def fun(arr, mid, l):
    n = len(arr)
    if mid < arr[0] or mid < l - arr[-1]:
        return False
    for i in range(n - 1):
        if not arr[i] + mid >= arr[i + 1] - mid:
            return False
    return True


def solve():
    n=iinp()
    arr=intl()
    i=0
    while i<n-1 and arr[i+1]>=arr[i]:
        i+=1
    if i==n-1:
        print(0)
        return
    ans=arr[i+1:]
    ans.extend(arr[:i+1])
    # print(ans,i)
    if sorted(arr)==ans:
        print(n-i-1)
    else:
        print(-1)



# -------------------------------------------------------Main Code------------------------------------------------------
start_time = time.time()
# for _ in range(iinp()):
solve()
# print("--- %s seconds ---" % (time.time() - start_time))
