#include <algorithm>
#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <cmath>
#include <utility>

using namespace std;
using ll = long long;
using ull = unsigned long long;

constexpr ll LLINF {1001002003004005006};//ll = 9*LLINF
constexpr int INTINF {1000000000};//int = 2*INTINF

#define  rep(i,n)   for(int i = 0; i < (n); ++i)
#define rrep(i,n)   for(int i = 1; i <= (n); ++i)
#define drep(i,n)   for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for(int i = s; i < t;  ++i)

template<typename T>
void maxs(T& x, T&& y) {
  x=std::max(x,y);
}
template<typename T>
void maxs(T& x, T& y) {
  x=std::max(x,y);
}

template<typename T>
void mins(T& x, T&& y) {
  x=std::min(x,y);
}
template<typename T>
void mins(T& x, T& y) {
  x=std::min(x,y);
}
#ifndef ATCODER_DSU_HPP
#define ATCODER_DSU_HPP 1

#include <algorithm>
#include <cassert>
#include <vector>

namespace atcoder {

// Implement (union by size) + (path compression)
// Reference:
// Zvi Galil and Giuseppe F. Italiano,
// Data structures and algorithms for disjoint set union problems
struct dsu {
  public:
    dsu() : _n(0) {}
    dsu(int n) : _n(n), parent_or_size(n, -1) {}
    int cnt;

    int merge(int a, int b) {
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        int x = leader(a), y = leader(b);
        if (x == y) return x;
        ++cnt;
        if (-parent_or_size[x] < -parent_or_size[y]) std::swap(x, y);
        parent_or_size[x] += parent_or_size[y];
        parent_or_size[y] = x;
        return x;
    }

    bool same(int a, int b) {
        assert(0 <= a && a < _n);
        assert(0 <= b && b < _n);
        return leader(a) == leader(b);
    }

    int leader(int a) {
        assert(0 <= a && a < _n);
        if (parent_or_size[a] < 0) return a;
        return parent_or_size[a] = leader(parent_or_size[a]);
    }

    int size(int a) {
        assert(0 <= a && a < _n);
        return -parent_or_size[leader(a)];
    }

    std::vector<std::vector<int>> groups() {
        std::vector<int> leader_buf(_n), group_size(_n);
        for (int i = 0; i < _n; i++) {
            leader_buf[i] = leader(i);
            group_size[leader_buf[i]]++;
        }
        std::vector<std::vector<int>> result(_n);
        for (int i = 0; i < _n; i++) {
            result[i].reserve(group_size[i]);
        }
        for (int i = 0; i < _n; i++) {
            result[leader_buf[i]].push_back(i);
        }
        result.erase(
            std::remove_if(result.begin(), result.end(),
                           [&](const std::vector<int>& v) { return v.empty(); }),
            result.end());
        return result;
    }

  private:
    int _n;
    // root node: -1 * component size
    // otherwise: parent
    std::vector<int> parent_or_size;
};

}  // namespace atcoder

#endif  // ATCODER_DSU_HPP

int main() {
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);

  int n, m;
  cin >> n >> m;
  vector<pair<int, int>> vector_data(m);
  rep(i, m) cin >> vector_data[i].first >> vector_data[i].second;
  atcoder::dsu union_find_tree(n);
  union_find_tree.cnt = 0;
  rep(i, m) {
    union_find_tree.merge(vector_data[i].first-1, vector_data[i].second-1);
  }
  std::cout << n - union_find_tree.cnt -1 << '\n';
}


Measured memory_bytes: 4520