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
# cf 454/B (1300)
def f(A):
    lo, hi = 0, len(A) - 1
    while lo < hi:
        mid = lo + (hi - lo) // 2
        # 1 2 3 4 5
        # b   m   e
        if A[mid] <= A[hi]:
            hi = mid
        else:
            lo = mid + 1
    assert lo == hi
    return lo

assert f([1, 2, 3, 4]) == 0
assert f([4, 1, 2, 3]) == 1
assert f([3, 4, 1, 2]) == 2
assert f([1, 3, 2]) == 2
assert f([2, 3, 4, 5, 6, 7, 8, 9, 10, 1]) == 9
assert f([99997, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]) == 1
assert f([1, 1, 1, 1, 1]) == 0
assert f([1, 1, 1, 1, 2, 0]) == 5
assert f([2, 3, 4, 1, 2]) == 3
assert f([5, 5, 5, 5, 3, 3, 3, 3, 4, 5]) == 4
assert f([5, 5, 5, 5, 5, 5, 5, 5, 3, 5, 5]) == 0 # !

n = int(input())
A = list(map(int, input().split()))
ans = 0

for i in range(1, len(A)):
    if A[i-1] > A[i]:
        ans = i
        break

if ans >= 0:
    for i in range(ans + 1, len(A)):
        if A[i-1] > A[i]:
            ans = -1
            break

if ans > 0:
    if A[-1] > A[0]:
        ans = -1

if ans > 0:
    print(len(A) - ans)
else:
    print(ans)
