// http://infoarena.ro/problema/dfs
#include <fstream>
#include <algorithm>
#include <vector>
#include <list>
using namespace std;

int nodes,edges,conexComponents;
vector<list<int>> graph;
vector<bool> visited;

void read();
void depthFirst(int startNode);
void write();

int main() {
	read();
	
	for(vector<bool>::iterator it=visited.begin();it<visited.end();it++) {
		if(*it) {
			depthFirst(*it);
			conexComponents++;
		}
	}

	write();

	return (0);
}

void read() {
	ifstream in("dfs.in");
	int from,to;

	in >> nodes >> edges;
	graph.resize(nodes+1);
	visited.resize(nodes+1);
	visited.at(0) = true;

	for(int i=1;i<=edges;i++) {
		in >> from >> to;

		graph.at(from).push_back(to);
		graph.at(to).push_back(from);
	}

	in.close();
}

void depthFirst(int toVisit) {
	visited.at(toVisit) = true;

	for(list<int>::iterator it=graph.at(toVisit).begin();it!=graph.at(toVisit).end();it++)
		if(!visited.at(*it))
			depthFirst(*it);
}

void write() {
	ofstream out("dfs.out");

	out << conexComponents-1 << "\n";

	out.close();
}