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
n = input()
l = map(int,raw_input().split())
l1 = []


for i in range(len(l)):
    l1.append(l[i])


max = l1[0]
m=0
count = 0
for i in range(1,len(l1)):
    if l1[i] >= max :

        max = l1[i]
        m=i

    if l[i]!=max:
        break
l.sort()

#print max
#print m
shifts = (len(l)-1)-m
#m1 = (len(l)-1)-m
for i in range(len(l)-1,-1,-1):
    #print m
    if l1[m] != l[i]:
        shifts = -1
        break
    shifts = shifts
    if m == 0:
        m = len(l)-1
    else:
        m=m-1

print shifts
