#include <fstream>
#define NMAX 1008
using namespace std;

ifstream fin("dfs.in");
ofstream fout("dfs.out");

int LA[NMAX][NMAX],viz[NMAX],n,m;

void citire()
{
	int i,x,y;
	fin>>n>>m;
	for (i=1; i<=m; i++)
		{
			fin>>x>>y;
			LA[x][++LA[x][0]]=y;
			LA[y][++LA[y][0]]=x;
		}
}

void dfs (int x)
{
	viz[x]=1;
	for (int i=1; i<=LA[x][0]; i++)
		if (viz[LA[x][i]]==0)
			dfs(LA[x][i]);
}

int main()
{
	int c=1;
	citire();
	for (int i=1; i<=n; i++)
		if (viz[i]==0)
			{
				c++;
				dfs(i);
			}
	fout<<c-1<<'\n';
	return 0;
}