#include <bits/stdc++.h>
#include <unistd.h>

#if __has_include(<local.h>)
//#include <local.h>
#endif

using namespace std;

typedef long long int LL;
typedef long long int LLint;

struct dsu {
  public:
    dsu() : _n(0) {}
    dsu(int n) : _n(n), parent_or_size(n, -1) {}

    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;
        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;
};

//Faster Program
#define endl "\n"

//Faster Coding
#define ED return
#define GET(a) int a;cin >> a
#define DGET(a) double a;cin >> a
#define SGET(a) string a;cin >> a
#define CGET(a) char a;cin >> a
#define PRINT(a) cout << a << "\n"
#define ANS(a,b) int a;cin >> a;cout << b << "\n"
#define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define SORT(vec) sort(vec.begin(),vec.end())
#define REV(vec) reverse(vec.begin(),vec.end())
#define ALL(x) (x).begin(),(x).end()

//As Function
#define UP(a,b) ((a+(b-1))/(b))
#define IPOW(x,y) LL(pow(x,y))

//Debug
#ifdef LOCAL_H
#define TEST cout << "OK\n"
#define DEBUG(a) a
#else
#define TEST
#define DEBUG(a)
#endif

#ifdef LOCAL_H
[[maybe_unused]]
#endif
const double PI = M_PI;
#ifdef LOCAL_H
[[maybe_unused]]
#endif
const int INF = int(999999999);
#ifdef LOCAL_H
[[maybe_unused]]
#endif
const LL LL_INF = LL(999999999999999999);
#ifdef LOCAL_H
[[maybe_unused]]
#endif
const long long MOD = int(1000000007);

//Settings
#ifdef LOCAL_H
const bool DO_BENCHMARK = true;
#endif
const int DOUBLE_ACCURACY = 15;
const int RUN_SOLVE = 1;

#ifndef LOCAL_H
const bool DO_BENCHMARK = false;
#endif

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(DOUBLE_ACCURACY);
    for(int do_solve_time = 0;do_solve_time < RUN_SOLVE;do_solve_time++){
        void solve();
        if(DO_BENCHMARK){
            clock_t clock_start = clock();
            solve();
            clock_t clock_end = clock();
            const double time = static_cast<double>(clock_end-clock_start)/CLOCKS_PER_SEC*1000.0;
            cout << "\n------------------------------------\n" << time << "[ms]\n";
        }
        else solve();
    }
}

/*------------------------------↓ここからコード↓----------------------------------*/
void solve(){
    int N, M;
    cin >> N >> M;
    dsu d(N);
    for(int i = 0;i < M;i++){
        int a, b;
        cin >> a >> b;
        d.merge(a-1, b-1);
    }
    vector<int> group (N);
    for(int i = 0;i < N;i++){
        group[i] = d.size(i);
    }
    SORT(group);
    int ans = -1;
    for(int i = 0;i < N;){
        ans++;
        i += group[i];
    }
    cout << ans << endl;
    return;
}


Measured memory_bytes: 3988