local n, m = io.read("*n", "*n")
local parent = {}
for i = 1, n do
  parent[i] = i
end

local function uf_findroot(idx)
  local idx_update = idx
  while parent[idx] ~= idx do
    idx = parent[idx]
  end
  while parent[idx_update] ~= idx do
    parent[idx_update], idx_update = idx, parent[idx_update]
  end
  return idx
end

for i = 1, m do
  local a, b = io.read("*n", "*n")
  local ra, rb = uf_findroot(a), uf_findroot(b)
  parent[rb], parent[b] = ra, ra
end
local g = {}
for i = 1, n do
  local r = uf_findroot(i)
  g[i] = true
end
local ret = -1
for k, v in pairs(g) do ret = ret + 1 end
print(ret)
