#include <bits/stdc++.h>

#ifdef DEBUG
#include <Mylib/Debug/debug.cpp>
#else
#define dump(...) ((void)0)
#endif

template <typename T, typename U>
bool chmin(T &a, const U &b){
  return (a > b ? a = b, true : false);
}

template <typename T, typename U>
bool chmax(T &a, const U &b){
  return (a < b ? a = b, true : false);
}

template <typename T, size_t N, typename U>
void fill_array(T (&a)[N], const U &v){
  std::fill((U*)a, (U*)(a + N), v);
}

template <typename T, size_t N, size_t I = N>
auto make_vector(const std::array<int, N> &a, T value = T()){
  static_assert(I >= 1);
  static_assert(N >= 1);
  if constexpr (I == 1){
    return std::vector<T>(a[N - I], value);
  }else{
    return std::vector(a[N - I], make_vector<T, N, I - 1>(a, value));
  }
}

template <typename T>
std::ostream& operator<<(std::ostream &s, const std::vector<T> &a){
  for(auto it = a.begin(); it != a.end(); ++it){
    if(it != a.begin()) s << " ";
    s << *it;
  }
  return s;
}

template <typename T>
std::istream& operator>>(std::istream &s, std::vector<T> &a){
  for(auto &x : a) s >> x;
  return s;
}

namespace haar_lib {
  class unionfind {
    int n;
    mutable std::vector<int> parent;
    std::vector<int> depth, size;
    int count;

  public:
    unionfind(){}
    unionfind(int n): n(n), parent(n), depth(n, 1), size(n, 1), count(n){
      std::iota(parent.begin(), parent.end(), 0);
    }

    int root_of(int i) const {
      if(parent[i] == i) return i;
      else return parent[i] = root_of(parent[i]);
    }

    bool is_same(int i, int j) const {return root_of(i) == root_of(j);}

    int merge(int i, int j){
      const int ri = root_of(i), rj = root_of(j);
      if(ri == rj) return ri;
      else{
        --count;
        if(depth[ri] < depth[rj]){
          parent[ri] = rj;
          size[rj] += size[ri];
          return rj;
        }else{
          parent[rj] = ri;
          size[ri] += size[rj];
          if(depth[ri] == depth[rj]) ++depth[ri];
          return ri;
        }
      }
    }

    int size_of(int i) const {return size[root_of(i)];}

    int count_groups() const {return count;}

    auto get_groups() const {
      std::vector<std::vector<int>> ret(n);

      for(int i = 0; i < n; ++i){
        ret[root_of(i)].push_back(i);
      }

      ret.erase(
        std::remove_if(
          ret.begin(), ret.end(),
          [](const auto &a){return a.empty();}
        ),
        ret.end()
      );

      return ret;
    }
  };
}


namespace haar_lib {}

namespace solver {
  using namespace haar_lib;

  constexpr int m1000000007 = 1000000007;
  constexpr int m998244353 = 998244353;

  void init(){
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    std::cout << std::fixed << std::setprecision(12);
    std::cerr << std::fixed << std::setprecision(12);
    std::cin.exceptions(std::ios_base::failbit);
  }

  void solve(){
    int N, M; std::cin >> N >> M;
    unionfind uf(N);
    for(int i = 0; i < M; ++i){
      int A, B; std::cin >> A >> B;
      --A, --B;
      uf.merge(A, B);
    }

    std::cout << uf.count_groups() - 1 << "\n";
  }
}

int main(){
  solver::init();
  while(true){
    try{
      solver::solve();
    }catch(const std::istream::failure &e){
      break;
    }
  }
  return 0;
}


Measured memory_bytes: 4520