import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
import java.util.Scanner;

class GRAPH {
    ArrayList<Integer> GR = new ArrayList<>();
}

public class BridgeNum {
   public static void DFS(GRAPH[] graph, int[] used, int[] par, Queue<Integer> queue, int a) {
        used[a] = 0;
        queue.add(a);
        for(int i = 0; i < graph[a].GR.size(); i++) {
            if(used[graph[a].GR.get(i)] == 1) {
                par[graph[a].GR.get(i)] = a;
                DFS(graph, used, par, queue, graph[a].GR.get(i));
            }
        }
    }

    public static int DFS2(GRAPH[] graph, int[] comp, int[] par, Queue<Integer> queue, int component) {
        while(queue.size() > 0) {
            int a = queue.remove();
            if(comp[a] == -1) {
                VisitVertex(graph, comp, par, a, component);
                component++;
            }
        }
        return component;
    }

    public static void VisitVertex(GRAPH[] graph, int[] comp, int[] par, int a, int component) {
        comp[a] = component;
        for(int i = 0; i < graph[a].GR.size(); i++) {
            if (comp[graph[a].GR.get(i)] == -1 && par[graph[a].GR.get(i)] != a)
                VisitVertex(graph, comp, par, graph[a].GR.get(i), component);
        }
    }
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n ,m;
        n = in.nextInt();
        int[] comp = new int[n];
        int[] par = new int[n];
        int[] used = new int[n];
        for(int i = 0; i < n; i++) {
            comp[i] = -1;
            par[i] = -1;
            used[i] = 1;
        }
        GRAPH[] G = new GRAPH[n];
        for(int i = 0; i < n; i++) {
            G[i] = new GRAPH();
        }
        m = in.nextInt();
        int u, v;
        for(int i = 0; i < m; i++) {
            u = in.nextInt();
            v = in.nextInt();
            G[v].GR.add(u);
            G[u].GR.add(v);
        }
        int component = 1;
        Queue<Integer> queue = new LinkedList<Integer>();
        for(int i = 0; i < n; i++) {
            if(used[i] == 1) {
                component--;
                DFS(G, used, par, queue, i);
            }
            component = DFS2(G, comp, par, queue, component);
        }
        System.out.println(component-1);
    }
}