local mfl, mce = math.floor, math.ceil
local mmi, mma = math.min, math.max
local bls, brs = bit.lshift, bit.rshift
local function lower_bound(ary, x)
  local num = #ary
  if x <= ary[1] then return 1 end
  if ary[num] < x then return num + 1 end
  local min, max = 1, num
  while 1 < max - min do
    local mid = brs(min + max, 1)
    if ary[mid] < x then
      min = mid
    else
      max = mid
    end
  end
  return max
end

local SegTree = {}
SegTree.clearAll = function(self)
  for i = self.stagenum, 1, -1 do
    local cnt = bls(1, i - 1)
    for j = 1, cnt do
      self.stage[i][j] = true
    end
  end
end
SegTree.create = function(self, n)
  local stagenum, mul = 1, 1
  self.stage = {{}}
  while mul < n do
    mul, stagenum = mul * 2, stagenum + 1
    self.stage[stagenum] = {}
  end
  self.stagenum = stagenum
  self:clearAll()
end
SegTree.setValue = function(self, idx)
  self.stage[self.stagenum][idx] = false
  for i = self.stagenum - 1, 1, -1 do
    local dst = brs(idx + 1, 1)
    local rem = dst * 4 - 1 - idx
    self.stage[i][dst] = self.stage[i + 1][idx] or self.stage[i + 1][rem]
    idx = dst
  end
end

SegTree.right_bound = function(self, left, right)
  local retpos = left - 1
  local stage, l, r = 1, left, right
  local stagenum = self.stagenum
  while true do
    local sz = bls(1, stagenum - stage)
    while (l - 1) % sz ~= 0 or r + 1 - l < sz do
      stage = stage + 1
      sz = bls(1, stagenum - stage)
    end
    if not self.stage[stage][mce(l / sz)] then
      retpos = l + sz - 1
      if retpos == right then break end
      if l + sz <= r then stage, l, r = 1, l + sz, r
      else break end
    else
      if sz ~= 1 then stage, l, r = stage + 1, l, l + sz - 2
      else break end
    end
  end
  return retpos + 1
end

SegTree.left_bound = function(self, left, right)
  local retpos = right + 1
  local stage, l, r = 1, left, bls(1, self.stagenum - 1)
  local stagenum = self.stagenum
  while true do
    local sz = bls(1, stagenum - stage)
    while r % sz ~= 0 or r + 1 - l < sz do
      stage = stage + 1
      sz = bls(1, stagenum - stage)
    end
    if not self.stage[stage][mfl(r / sz)] then
      retpos = r - sz + 1
      if l + sz <= r then stage, l, r = 1, l, r - sz
      else break end
    else
      if sz ~= 1 then stage, l, r = stage + 1, r - sz + 2, r
      else break end
    end
  end
  return retpos - 1
end
SegTree.new = function(n)
  local obj = {}
  setmetatable(obj, {__index = SegTree})
  obj:create(n)
  return obj
end

local n, m = io.read("*n", "*n")
local a = io.read("*n")
local t = {}
n = n - 1
for i = 1, n do
  t[i] = io.read("*n")
end
table.sort(t)
local st = SegTree
st:create(n)

local function judge(x)
  local tgt = a + t[x] + 1
  st:clearAll()
  st:setValue(x)
  local cnt = 0
  for i = 1, m do
    local rightpos = st:left_bound(1, n)
    if rightpos <= 1 then return true end
    st:setValue(rightpos)
    local lbpos = lower_bound(t, tgt - t[rightpos])
    if n < lbpos then return true end
    local leftpos = st:right_bound(lbpos, n)
    if n < leftpos then return true end
    st:setValue(leftpos)
  end
  return false
end
if not judge(n) then
  print(-1)
elseif judge(1) then
  print(t[1])
else
  local min, max = 1, n
  while 1 < max - min do
    local mid = brs(min + max, 1)
    if judge(mid) then
      max = mid
    else
      min = mid
    end
  end
  print(t[max])
end