#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
#include <array>
#include <vector>
#include <deque>
#include <list>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <stack>
#include <queue>
#include <bitset>
#include <tuple>
#include <cmath>
#include <complex>
#include <algorithm>
#include <utility>
#include <regex>
#include <cstdint>
#include <numeric>
#include <functional>
#include <cassert>

using namespace std;

namespace utils{
#define ALL(x) begin(x), end(x)
#define RALL(x) rbegin(x), rend(x)
    ///----- aliases
    using ll = long long int;
    using ull = unsigned long long;

    template<class T, class Compare> using p_queue = priority_queue<T, vector<T>, Compare>;
    template<class T> using min_queue = p_queue<T, greater<T>>;
    template<class T> using max_queue = p_queue<T, less<T>>;

    template<class T> inline bool CHMIN(T& X, const T& A){ if(X > A) {X = A; return true;} return false; }
    template<class T> inline bool CHMAX(T& X, const T& A){ if(X < A) {X = A; return true;} return false; }

    ///----- vector I/O
    template<class T>
    vector<T> VEC(size_t n, T t){
        return vector<T>(n, t);
    }

    template<class ...Ts>
    auto VEC(size_t n, Ts ... ts){
        return vector<decltype(VEC(ts...))>(n, VEC(ts...));
    }


    template<class T>
    istream& operator>>(istream& is, vector<T>& v){
        for (auto &&x : v) { is >> x; } return is;
    }

    template<class T>
    ostream& operator<<(ostream& os, const vector<T>& v){
        auto p = v.begin();
        assert(p != v.end());
        os << *p++;
        while(p != v.end()){
            os << ' ' << *p++;
        }
        return os ;
    }

    template<class T>
    ostream& operator<<(ostream& os, const vector<vector<T>>& v){
        auto p = v.begin();
        assert(p != v.end());
        os << *p++;
        while(p != v.end()){
            os << '\n' << *p++;
        }
        return os;
    }

    ///----- tuple I/O
    template <class S, class T>
    istream& operator>>(istream& is, tuple<S, T>& t){
        return is >> get<0>(t) >> get<1>(t);
    }

    template <class S, class T>
    istream& operator>>(istream& is, pair<S, T>& t){
        return is >> get<0>(t) >> get<1>(t);
    }

    template <class S, class T, class U>
    istream& operator>>(istream& is, tuple<S, T, U>& t){
        return is >> get<0>(t) >> get<1>(t) >> get<2>(t);
    }

    template <class S, class T>
    ostream& operator<<(ostream& os, const tuple<S, T>& t){
        return os << get<0>(t) << ' ' <<  get<1>(t);
    }

    template <class S, class T>
    ostream& operator<<(ostream& os, const pair<S, T>& t){
        return os << get<0>(t) << ' ' <<  get<1>(t);
    }

    template <class S, class T, class U>
    ostream& operator<<(ostream& os, const tuple<S, T, U>& t){
        return os << get<0>(t) << ' ' <<  get<1>(t) << ' ' <<  get<2>(t);
    }

    ///----- constants
    constexpr ll INFLL   = 1'000'000'000'000'000'020ll;
    constexpr ll INF     = 1'000'000'009;
    constexpr double PI      = 3.14'159'265'358'979'323'846;
    constexpr double EPS     = 1e-12;
}
using namespace utils;

class UFF {
    using Int = int;
    std::vector<Int> parent;
    std::vector<Int> rank;

public:
    explicit UFF(Int n) : parent(n, -1), rank(n, 0) {}

    Int find(Int x) {
        auto &px = parent[x];
        if (px < 0) {
            return x;
        } else {
            return px = find(px);
        }
    }

    bool unite(Int a, Int b) {
        const auto pa = find(a);
        const auto pb = find(b);
        if (pa == pb) return false;

        if (rank[pa] > rank[pb]) {
            parent[pa] += parent[pb];
            parent[pb] = pa;
        } else {
            parent[pb] += parent[pa];
            parent[pa] = pb;

            if (rank[pa] == rank[pb]) {
                rank[pb]++;
            }
        }
        return true;
    };

    Int size(Int x) {
        const auto px = find(x);
        return -parent[px];
    }

    bool is_same(Int a, Int b) {
        const auto pa = find(a);
        const auto pb = find(b);
        return pa == pb;
    }

    vector<Int> get_all_dset() {
        unordered_set<Int> dset;
        for (Int i = 0; i < parent.size(); ++i) {
            dset.insert(find(i));
        } // end i
        vector<Int> v(dset.size());
        Int i = 0;
        for (auto &&d : dset) {
            v[i++] = d;
        } // end d
//        sort(v.begin(), v.end());
        return v;
    }
};


class solver{
    istream& is;
    ostream& os;

public:
    solver(istream& I, ostream& O) :is(I), os(O) {}


    void run();
};

void solver::run(){
    int N, M;
    is >> N >> M;
    UFF uf(N);
    for (int i = 0; i < M; ++i) {
        int a , b;
        is >> a >> b;
        a--; b--;
        uf.unite(a, b);
    } // end i
    os << uf.get_all_dset().size() -1 << endl;
}

int main(int argc, char *argv[]) {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << setprecision(16) << scientific;
#ifdef XELMH
    string test_cases = "test_c.txt";
    cerr << test_cases << " -->" << endl;
    auto fs = fstream(test_cases, fstream::in);
    int loop = 0;
    while(fs) {
        loop++;
        cout << '#' << loop << "#------\n";
        solver(fs, cout).run();
    }
    if(loop <= 1) {
        cout << "===" << endl;
        while(cin) solver(cin, cout).run();
    }
#else
    solver(cin, cout).run();
#endif
    return 0;
}


Measured memory_bytes: 7784