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
from sys import stdin,stdout
from collections import Counter
def ai(): return list(map(int, stdin.readline().split()))
def ei(): return map(int, stdin.readline().split())
def ip(): return  int(stdin.readline().strip())
def op(ans): return stdout.write(str(ans) + '\n')
from math import ceil


n = ip()
li = ai()
x = sorted(li)
if x==li:exit(print(0))
c = 0
for i in range(n-1):
    if li[i+1]<li[i]:
        c = i+1
ans = li[c:]+li[:c]
for i in range(n-1):
    if ans[i]>ans[i+1]:
        print(-1)
        break
else:
    print(n-c)