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