local mmi, mma = math.min, math.max
local inf = 1000000007
local n, m = io.read("*n", "*n")
local a, b = io.read("*n", "*n")
local t = {}
for i = 1, a do
  t[i] = 0
end
for i = a + 1, n + 1 do
  t[i] = inf
end
local edge = {}
for i = 1, n + 1 do
  edge[i] = {}
end
for i = 1, m do
  local l, r = io.read("*n", "*n")
  edge[l][r + 1] = true
  edge[r + 1][l] = true
end
local tasks = {}
for i = 1, a do
  tasks[i] = i
end
local done = 0
while done < #tasks do
  done = done + 1
  local src = tasks[done]
  for dst, _u in pairs(edge[src]) do
    if 1 + t[src] < t[dst] then
      t[dst] = 1 + t[src]
      table.insert(tasks, dst)
    end
  end
end
local ret = inf
for i = b + 1, n + 1 do
  ret = mmi(ret, t[i])
end
if ret == inf then ret = -1 end
print(ret)