#{{{ header
import algorithm, sequtils, tables, macros, math, sets, strutils
when defined(MYDEBUG):
  import header

proc scanf(formatstr: cstring){.header: "<stdio.h>", varargs.}
proc getchar(): char {.header: "<stdio.h>", varargs.}
proc nextInt(): int = scanf("%lld",addr result)
proc nextFloat(): float = scanf("%lf",addr result)
proc nextString(): string =
  var get = false
  result = ""
  while true:
    var c = getchar()
    if int(c) > int(' '):
      get = true
      result.add(c)
    else:
      if get: break
      get = false
template `max=`*(x,y:typed):void = x = max(x,y)
template `min=`*(x,y:typed):void = x = min(x,y)
template inf(T): untyped = ((T(1) shl T(sizeof(T)*8-2)) - 1)
#}}}


#{{{ Graph
import sequtils

type
  Edge[T] = object
    src,dst:int
    weight:T
    rev:int
  Edges[T] = seq[Edge[T]]
  Graph[T] = seq[seq[Edge[T]]]

proc initEdge[T](src,dst:int,weight:T,rev:int = -1):Edge[T] =
  var e:Edge[T]
  e.src = src
  e.dst = dst
  e.weight = weight
  e.rev = rev
  return e

proc initGraph[T](n:int):Graph[T] =
  return newSeqWith(n,newSeq[Edge[T]]())

proc addBiEdge[T](g:var Graph[T],src,dst:int,weight:T=1):void =
  g[src].add(initEdge(src,dst,weight,g[dst].len))
  g[dst].add(initEdge(dst,src,weight,g[src].len-1))

proc addEdge[T](g:var Graph[T],src,dst:int,weight:T=1):void =
  g[src].add(initEdge(src,dst,weight,-1))

proc `<`[T](l,r:Edge[T]):bool = l.weight < r.weight
#}}}

var v = newSeq[int]()

# depth first search {{{
proc dfs(g:Graph[int], u:int, p = -1, h = 0) =
  v.add(h)
  for e in g[u]:
    if e.dst == p: continue
    g.dfs(e.dst, u, h + 1)
#}}}

block main:
  let N, K = nextInt()
  if K > N: echo -1;break
  else: echo K - 1
#  var g = initGraph[int](N)
#  for i in 0..<N-1:
#    let a, b = nextInt() - 1
#    g.addBiEdge(a, b)
#  g.dfs(0)
#  v.sort()
#  echo v[0..<K].sum
  discard