E

#include<bits/stdc++.h>
#define ll long long
#define endl "\n"
using namespace std;
template<class T>
void printff(vector<T>& v) {
  for (auto k : v) cout << k << " ";
  cout << endl;
}
void SOLVE() {
  int n, m; cin >> n >> m;
  vector<vector<int>> graph(n + 1);
  vector<bool> visited(n + 1);
  for(int i = 0, u, v; i < m; i++){
    cin >> u >> v;
    graph[v].push_back(u);
    graph[u].push_back(v);
  }
  function<void(int)> dfs =[&] (int node){
    visited[node] = true;
    for(auto child : graph[node]){
        if(!visited[child]) dfs(child);
    }
  };
  int counter = 0;
  for(int i = 1; i <= n; i++){
    if(!visited[i]){
        dfs(i);
        counter++;
    }
  }
  cout << counter - 1 << endl;
}
int main() {
  ios_base::sync_with_stdio(false); 
  cin.tie(NULL); 
  cout.tie(NULL);
  int tc = 1; //cin >> tc;
  while(tc--) SOLVE();
  return 0;
}