#include <fstream>
#include <vector>
#include <queue>
using namespace std;
ifstream in("dfs.in");
ofstream out("dfs.out");
int n,m,k,r,x,y;
queue <int> q;
struct vecini
{ int n;
  vector <int> v;
};
int main()
{ in>>n>>m;
  int v[n+1]={0};
  vecini a[n+1]={0};
  for(int i=1;i<=m;i++)
  { in>>x>>y;
    a[x].v.push_back(y);
    a[x].n++;
    a[y].v.push_back(x);
    a[y].n++;
  }
  y=1;
  while(k<n)
  { r++;
    for(int i=y;i<=n;i++)
      if(v[i]==0)
         q.push(i),v[i]=r,i=n+1,k++;
    y=q.front();
    while(!q.empty())
    { x=q.front();
      q.pop();
      for(int i=0;i<a[x].n;i++)
      { if(!v[a[x].v[i]])
        { v[a[x].v[i]]=r;
          q.push(a[x].v[i]);
          k++;
        }
      }
    }
  }
  out<<r;
  in.close();
  out.close();
  return 0;
}