n,k = map(int,input().split())
arr= list(map(int,input().split()))
ans = k-1
flag = 0
for i in range(k-1,n-1):
    if arr[i]!=arr[i+1]:
        flag = 1
        break
arr.sort()
if arr[0]==arr[-1]:
    print(0)
    quit()
if flag == 1:
    print(-1)
else:
    print(ans)