#include <bits/stdc++.h>
using namespace std;

#define endl '\n'
#define int long long
#define pii pair <int, int>
#define lc(i) (i << 1)
#define rc(i) (i << 1 | 1)
#define MAXN 100005
#define MAXM 1000005
int mod = 100000007;

int n, m;
int root[MAXN];//root[i]: i 点所在块的根节点

int search(int node) {
	if (root[node] == node)
		return  node;
	return root[node] = search(root[node]);//优化
}
signed main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);

//	freopen(".in", "r", stdin);
//	freopen(".out", "w", stdout);

	int n, m;
	cin >> n >> m;
	for (int i = 1; i <= n; ++i)//初始化，每一个点为一块
		root[i] = i;
	for (int i = 1; i <= m; ++i) {
		int u, v;
		cin >> u >> v;
		root[search(u)] = root[search(v)];//合并
	}
	int div = 0;//块数量
	for (int i = 1; i <= n; ++i) {
		if (search(i) == i) 
			++div;
	}
	cout << div - 1 << endl;
	return 0;
}