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
# Aditya Morankar
# lsta = list(map(int,input().split()))
def main():
    n = int(input())
    lst = list(map(int,input().split()))
    
    ispeak=0
    indexx=-1
    for i in range(1,n):
        if lst[i] < lst[i-1]:
            ispeak += 1
            if ispeak==1:
                indexx = i
    
    if ispeak==0:
        print(0)
    elif ispeak!=1:
        print(-1)
    else:
        if lst[0] >= lst[n-1]:
            print(n-indexx)
        else: print(-1)
            
        
        
    
    
    
    

if __name__ == '__main__':
    t = 1
    while t!=0:
        main()
        t-=1