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
"""
http://codeforces.com/contest/454/problem/B
"""
import sys
N = int(raw_input())
A = map(int,raw_input().split())
A=A+[A[0]]
#print N,A
fails = 0
rots = 0
for i in xrange(N) :
	if A[i] <= A[i+1] :
		rots+=1
	elif fails == 1 :
		print -1
		sys.exit(0)
	else :
		fails+=1
		rots = 0
if i==N-1 and fails == 1:
	print rots
else :
	print 0
