#pragma GCC optimize("O3", "unroll-loops")
#pragma GCC target("sse4.2")

#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <map>
#include <unordered_map>
#include <set>
#include <unordered_set>
#include <bitset>
#include <sstream>
#include <deque>
#include <queue>
#include <complex>
#include <random>
#include <cassert>
#include <chrono>

using namespace std;

#define FAST ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define FIXED cout << fixed << setprecision(12)
#define ll long long
#define ld long double
#define pii pair<int, int>
#define pll pair<ll, ll>
#define graph vector<vector<int>>
#define pb push_back
#define pf push_front
#define popb pop_back
#define popf pop_front
#define f first
#define s second
#define hashmap unordered_map
#define hashset unordered_set
#define eps 1e-9
#define mod 1000000007
#define inf 3000000000000000007ll
#define sz(a) signed((a).size())
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()

#ifdef DEBUG
    mt19937 gen(857204);
#else
    mt19937 gen(chrono::high_resolution_clock::now().time_since_epoch().count());
#endif

#ifdef DEBUG
    template<class T> T to_dbg(T x) { return x; }
    template<class T, class U> string to_dbg(pair<T, U> p) {
        stringstream ss;
        ss << '{' << p.f << ',' << p.s << '}';
        return ss.str();
    }
    string to_dbg(string s) { return "\"" + s + "\""; }
    template<class It> string to_dbg(It begin, It end, string d = "") {
        stringstream ss;
        ss << '{';
        if (begin != end) ss << to_dbg(*begin++);
        while (begin != end)
            ss << "," << d << to_dbg(*begin++);
        ss << '}';
        return ss.str();
    }
    template<class T> string to_dbg(vector<T> a) { return to_dbg(all(a)); }
    template<class T> string to_dbg(set<T> a) { return to_dbg(all(a)); }
    template<class T> string to_dbg(hashset<T> a) { return to_dbg(all(a)); }
    template<class T, class U> string to_dbg(map<T, U> a) { return to_dbg(all(a), "\n"); }
    template<class T, class U> string to_dbg(hashmap<T, U> a) { return to_dbg(all(a), "\n"); }
    template<class T> void dbgout(T x) { cout << to_dbg(x) << endl; }
    template<class T, class... U>
    void dbgout(T t, U... u) {
        cout << to_dbg(t) << ", ";
        dbgout(u...);
    }
    #define dbg(...) cout << "[" << #__VA_ARGS__ << "] = ", dbgout(__VA_ARGS__)
#else
    #define dbg(...) 0
#endif

template<class T, class U> inline bool chmax(T &x, U y) { return x < y ? x = y, 1 : 0; }
template<class T, class U> inline bool chmin(T &x, U y) { return x > y ? x = y, 1 : 0; }
template<class T> inline void sort(T &a) { sort(all(a)); }
template<class T> inline void rsort(T &a) { sort(rall(a)); }
template<class T> inline void reverse(T &a) { reverse(all(a)); }
template<class T, class U> inline istream& operator>>(istream& str, pair<T, U> &p) { return str >> p.f >> p.s; }
template<class T> inline istream& operator>>(istream& str, vector<T> &a) { for (auto &i : a) str >> i; return str; }
template<class T> inline T sorted(T a) { sort(a); return a; }
void read() {} void print() {} void println() { cout << '\n'; }
template<class T, class ...U> void read(T &x, U& ... u) { cin >> x; read(u...); }
template<class T, class ...U> void print(const T &x, const U& ... u) { cout << x; print(u...); }
template<class T, class ...U> void println(const T &x, const U& ... u) { cout << x; println(u...); }

struct dsu {
    struct rnk {
        int val = 1;
        int get() const {
            return val;
        }
        void add(const rnk &r) {
            val += r.val;
        }
    };
    vector<int> p;
    vector<rnk> r;
    int comp = 0, edge = 0;
    dsu() {}
    bool empty() const {
        return p.empty();
    }
    dsu(int n) {
        p = vector<int>(n);
        r = vector<rnk>(n);
        for (int i = 0; i < n; ++i)
            p[i] = i;
        comp = n;
    }
    int getp(int v) { // returns head element in a set
        return v == p[v] ? v : p[v] = getp(p[v]);
    }
    bool check(int a, int b) { // a and b are in one component
        return getp(a) == getp(b);
    }
    bool unite(int a, int b) { // a and b are in diff components
        a = getp(a);
        b = getp(b);
        if (a == b) return false;
        ++edge;
        --comp;
        if (r[a].get() > r[b].get()) swap(a, b);
        p[a] = b;
        r[b].add(r[a]);
        return true;
    }
};

signed main() {
    FAST; FIXED;
    int n;
    read(n);
    dsu d(n);
    int m;
    read(m);
    while (m--) {
        int u, v;
        read(u, v);
        d.unite(u - 1, v - 1);
    }
    println(d.comp - 1);
    #ifdef DEBUG
        cerr << "Runtime is: " << clock() * 1.0 / CLOCKS_PER_SEC << endl;
    #endif
    return 0;
}

Measured memory_bytes: 4020