#include <stack>
#include <vector>
#include <fstream>
#include <algorithm>

using namespace std;

const int Max = 100001;
int noNodes, noEdges;
bool Mark[Max];
vector<int> Adj[Max];

void readData (fstream &in){
	int _nodeA, _nodeB;
	in >> noNodes >> noEdges;
	for (int i = 1; i <= noEdges; ++i){
		in >> _nodeA >> _nodeB;
		Adj[_nodeA].push_back(_nodeB);
		Adj[_nodeB].push_back(_nodeA);
	}
	in.close();
}

void depthFirstSearch (int nextNode){
	Mark[nextNode] = true;
	for (int i = 0; i < Adj[nextNode].size(); ++i){
		if (!Mark[Adj[nextNode][i]]){
			depthFirstSearch(Adj[nextNode][i]);
		}
	}
}


void findConexityComponents (fstream &out){
	int nextNode, countConexity;
	countConexity = 1;
	for (int i = 1; i <= noNodes; ++i){
		if (!Mark[i]){
			nextNode = i;
			depthFirstSearch(nextNode);
			countConexity++;
		}
	}
	out << --countConexity;
	out.close();
}

int main (int argc, char *argv[]){
	fstream in("dfs.in", ios::in);
	fstream out("dfs.out", ios::out);
	readData(in);
	findConexityComponents(out);
	return 0;
}