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: lenovo
'''

import sys

# sys.stdin = open("input.txt", "r")

n = int(raw_input())
li = [ int(x) for x in raw_input().split() ]

idx = 1

while idx < n and li[idx - 1] <= li[idx]:
    idx += 1
    
if idx == n:
    print (0)
else:
    idx1 = idx
    ok = True
    while idx1 + 1 < n and li[idx1] <= li[0] and li[idx1] <= li[idx1 + 1]:
        idx1 += 1
    if idx1 >= n - 1 and li[idx1] <= li[0]:
        res = idx1 - idx + 1
        print (res)
    else:
        print (-1)