text: n,m = [int(i) for i in input().split()]
  arr = [int(i) for i in input().split()]
  j =[]
  for i in range(len(arr)-1,0,-1):
      if arr[i] == m:
          j.append(i+1)
          break
  if j[-1]==0:
      print(-1)
  else:
      print(j[-1])
  