#include <bits/stdc++.h>
using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) {int v; istringstream sin(s);sin>>v;return v;}
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}

//math
//-------------------------------------------
template<class T> inline T sqr(T x) {return x*x;}

//typedef
//------------------------------------------
typedef long long LL;
typedef pair<int, int> PII;
typedef pair<LL,LL> PLL;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef vector<bool> VB;
typedef vector<VB> VVB;
typedef vector<double> VD;
typedef vector<VD> VVD;
typedef vector<string> VS;
typedef vector<VS> VVS;
typedef vector<char> VC;
typedef vector<VC> VVC;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
typedef priority_queue<int> PQGI; //大きい順
typedef priority_queue<int, VI, greater<int>> PQLI;
typedef priority_queue<PII> PQGP;
typedef priority_queue<PII, VPII, greater<PII>> PQLP;
//priority_queue<T,vector<T>,decltype(f)> pq{f};

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EB emplace_back
#define EF emplace_front
#define PB push_back
#define PF push_front
#define POB pop_back
#define POF pop_front
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define SORTR(c) sort((c).rbegin(), (c).rend())
#define LB lower_bound
#define UB upper_bound
#define NEXP next_permutation
#define FI first
#define SE second
#define Vmin(a) *min_element((a).begin(),(a).end())
#define Vmax(a) *max_element((a).begin(),(a).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define FORR(i,a,b) for(int i = (b-1);i>=(a);i--)
#define REPR(i,n) FORR(i,0,n)
#define CFOR(i,a,b) for(int i=(a);i<=(b);++i)
#define CREP(i,n)  CFOR(i,0,n)
#define CFORR(i,a,b) for(int i = (b);i>=(a);i--)
#define CREPR(i,n) CFORR(i,0,n)
#define BFOR(bit,a,b) for(int bit = (a); bit < (1<<(b)); ++bit)
#define BREP(bit,n) BFOR(bit,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int INF = INT_MAX/2;
const LL LINF = LLONG_MAX/3;
const int RINF = INT_MIN/2;
const LL RLINF = LLONG_MIN/3;
const LL MOD = 1e9+7;
const LL MODD = 998244353;
const int MAX = 510000;

inline bool Eq(double a, double b) { return fabs(b - a) < EPS; }

//other
//-------------------------------------------
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

#define COUT(x) cout << (x) << endl
#define COUT2(x,y) cout << (x) << " " << (y) << endl
#define COUT3(x,y,z) cout << (x) << " " << (y) << " " << (z) << endl
#define PR(x) cout << (x)
#define ENDL cout << endl
#define SPACE cout << " "
#define BC(x) __builtin_popcountll(x)

VI dx = {1,0,-1,0,1,1,-1,-1};
VI dy = {0,1,0,-1,1,-1,1,-1};

LL Gcd(LL a, LL b) {return __gcd(a,b);}
LL Lcm(LL a, LL b) {return a/Gcd(a,b)*b;}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 >& p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < SZ(v); i++) {
    os << v[i] << (i + 1 != SZ(v) ? " " : "");
  }
  return os;
}

template<typename T>
vector<T> make_v(size_t a,T b){return vector<T>(a,b);}
template<typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v(ts...))>(a,make_v(ts...));
}

struct UnionFind {
  
  VI par, siz;
  int cnt,n;
  
  UnionFind() {}
  
  UnionFind(int n_):par(n_), siz(n_,1),cnt(n_),n(n_) {
    iota(ALL(par),0);
  }
  
  void init(int n_) {
    par.assign(n_,0);
    siz.assign(n_,1);
    iota(ALL(par),0);
    cnt = n_;
    n = n_;
  }
  
  int find(int x) {
    if(par[x] == x){
      return x;
    }else{
      return par[x] = find(par[x]);
    }
  }

  bool unite(int x, int y){
    x = find(x);
    y = find(y);
    if(x == y) return false;
    cnt--;

    if(siz[x] < siz[y]){
      swap(x,y);
    }
    siz[x] += siz[y];
    par[y] = x;
    return true;
    }

  bool same(int x, int y){
    return find(x) == find(y);
  }
  
  int size(int x){
    return siz[find(x)];
  }


  VVI groups() {
    VI leader_buf(n), group_size(n);
    REP(i,n){
      leader_buf.at(i) = find(i);
      group_size.at(leader_buf.at(i))++;
    }
    VVI result(n);
    REP(i,n){
      result.at(i).reserve(group_size.at(i));
    }
    REP(i,n){
      result.at(leader_buf.at(i)).EB(i);
    }
    result.erase(
      remove_if(
        ALL(result),
        [&](const VI& v) { return v.empty(); }
      ),
      result.end()
    );
    return result;
  }

};


int main() {

  // cin.tie(0);
  // ios::sync_with_stdio(false);
  cout << fixed << setprecision(12);

  int N, M;
  cin >> N >> M;

  UnionFind uf(N);

  REP(i,M){
    int a,b;
    cin >> a >> b;
    a--; b--;
    uf.unite(a,b);
  }

  COUT(uf.cnt-1);

  return 0;
}

Measured memory_bytes: 4048