Problem:
Score : 300 points

Problem Statement

There are N cities numbered 1 through N, and M bidirectional roads numbered 1 through M.
Road i connects City A_i and City B_i.

Snuke can perform the following operation zero or more times:

Choose two distinct cities that are not directly connected by a road, and build a new road between the two cities.

After he finishes the operations, it must be possible to travel from any city to any other cities by following roads (possibly multiple times).

What is the minimum number of roads he must build to achieve the goal?

Constraints

2 \leq N \leq 100,000

1 \leq M \leq 100,000

1 \leq A_i < B_i \leq N

No two roads connect the same pair of cities.

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
:
A_M B_M

Output

Print the answer.

Sample Input 1

3 1
1 2

Sample Output 1

1

Initially, there are three cities, and there is a road between City 1 and City 2.

Snuke can achieve the goal by building one new road, for example, between City 1 and City 3.
After that,

We can travel between 1 and 2 directly.

We can travel between 1 and 3 directly.

We can travel between 2 and 3 by following both roads (2 - 1 - 3).

Solution in C++ (GCC 9.2.1 with AC Library v1.1) (judge verdict: Accepted)

#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
using namespace std;
using ll = long long;

struct UnionFind {
private:
  vector<int> child, tree;
  vector<vector<int>> list;

public:
  //頂点数v
  UnionFind(int v) {
    tree.resize(v);
    list.resize(v);
    for(int i = 0; i < v; i++) {
      tree[i] = i;
      list[i].push_back(i);
    }
  }

  //木の根を求める
  int root(int x) {
    if(x == tree[x]) {
      for(int i = 0; i < child.size(); i++) {
        tree[child[i]] = x;
      }
      child.clear();
      return x;
    } else {
      child.push_back(x);
      return x = root(tree[x]);
    }
  }
  // xの所属するグループの大きさ
  int size(int x) { return list[root(x)].size(); }

  vector<int> nodes(int no) { return list[root(no)]; }

  // xとyの集合を合わせる
  bool unit(int x, int y) {
    x = root(x);
    y = root(y);
    if(x == y) {
      return false;
    }
    if(list[x].size() < list[y].size()) {
      swap(x, y);
    }
    for(int no : list[y]) {
      list[x].emplace_back(no);
    }
    tree[y] = x;
    return true;
  }

  // xとyが同じグループに所属するか
  bool isUnit(int x, int y) { return root(x) == root(y); }
};
int main() {
  int n, m;
  cin >> n >> m;
  UnionFind uf(n);
  for(int i = 0; i < m; i++) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    uf.unit(a, b);
  }
  int ans = 0;
  for(int i = 1; i < n; i++) {
    if(uf.isUnit(0, i) == false) {
      uf.unit(0, i);
      ans++;
    }
  }
  cout << ans << endl;
  return 0;
}