#include <bits/stdc++.h>
#define ll long long
#define nxM 200010
#define mxN 100010
using namespace std;

int n, m, ans = 0;
vector<int> edges[nxM];
stack<int> s;
bool visited[mxN];

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

void dfs() {
    while(!s.empty()) {
        ++ans;
        int node = s.top();
        s.pop();
        for (int i = 0; i < edges[node].size(); ++i) {
            if (visited[edges[node][i]] == 0) {
                visited[edges[node][i]] = 1;
                s.push(edges[node][i]);
            }
        }
    }
}

int main() {
    fin.tie(0);
    ios::sync_with_stdio(0);
    fin >> n >> m;
    for (int i = 1; i <= m; ++i) {
        int x, y;
        fin >> x >> y;
        edges[x].push_back(y);
        edges[y].push_back(x);
    }
    s.push(1);
    dfs();
    fout << ans - 1;
	return 0;
}