local n, m = io.read("*n", "*n")
local d = {}
for i = 1, n do
  d[i] = {}
  for j = 1, m do
    d[i][j] = io.read("*n")
  end
  table.sort(d[i])
end
local dp1, dp2 = {}, {}
local function solve(x)
  for i = 1, m + 1 do
    dp1[i] = true
  end
  for i = 2, n do
    local src = i % 2 == 0 and dp1 or dp2
    local dst = i % 2 == 0 and dp2 or dp1
    for j = 1, m + 1 do
      dst[j] = 0
    end
    local dsrc = d[i - 1]
    local ddst = d[i]
    local bleft, bright = 1, 1
    for apos = 1, m do
      while bleft <= m and ddst[bleft] < dsrc[apos] do
        bleft = bleft + 1
      end
      if m < bleft then break end
      while bright <= m and ddst[bright] <= dsrc[apos] + x do
        bright = bright + 1
      end
      if src[apos] then
        dst[bleft] = dst[bleft] + 1
        dst[bright] = dst[bright] - 1
      end
    end
    for j = 2, m do
      dst[j] = dst[j] + dst[j - 1]
    end
    local f = false
    for j = 1, m do
      dst[j] = 0 < dst[j]
      if dst[j] then f = true end
    end
    if not f then return false end
  end
  return true
end
local min, max = -1, 1000000000
while 1 < max - min do
  local mid = math.floor((min + max) / 2)
  if solve(mid) then
    max = mid
  else
    min = mid
  end
end
if max == 1000000000 then print(-1)
else print(max)
end