#ifdef LOCAL
#include <header.hpp>
#else
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#endif
#define lp(i, l, r) for (auto i = regu(l); i <= decltype(i)(r); ++i)
#define rp(i, r, l) for (auto i = regu(r); i >= decltype(i)(l); --i)
#define lv(i, a) for (int i = 0; i < int(a.size()); ++i)
#define rv(i, a) for (int i = int(a.size()) - 1; i >= 0; --i)
#define lm(i, m) for (auto i = m; i; i = (m) & (i - 1))
#define many for (int C = 1, T = -1; T < 0 && cin >> T, C <= T; ++C)
#define goog "Case #" + to_string(C) + ":"
#define rang(a) begin(a), end(a)
#define elif else if
#ifndef LOCAL
#define endl '\n'
#undef assert
#define assert
#endif
using namespace std;
using namespace __gnu_pbds;
const auto eps = 1e-8, pi = acos(-1);
template <class T> int comp(const T &a, const T &b) {
    return a < b - eps ? -1 : (a > b + eps);
}
#if __cplusplus > 201103L
template <class T> auto regu(const T &a) { return int(a); }
template <> auto regu(const int64_t &a) { return a; }
template <> auto regu(const uint64_t &a) { return int64_t(a); }
#else
template <class T> int64_t regu(const T &a) { return int(a); }
template <> int64_t regu(const int64_t &a) { return a; }
template <> int64_t regu(const uint64_t &a) { return a; }
#endif
template <class T> int barr(const T &a, const int &i) {
    return int(a >> i & 1);
}
template <class T> int bcnt(const T &a) { return __builtin_popcount(a); }
template <> int bcnt(const int64_t &a) { return __builtin_popcountll(a); }
template <class T> int blen(const T &a) { return a ? 1 + blen(a / 2) : 0; }
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
template <class T> T rint(T l, T r) {
    return uniform_int_distribution<T>(l, r)(rng);
}
template <class A, class B> B conv(const A &a, B b = B()) {
    stringstream s;
    s << a, s >> b;
    return b;
}
template <class T = int> T inf() { return numeric_limits<T>::max() / 2; }
template <class T> T sign(const T &a) { return a == 0 ? 0 : (a < 0 ? -1 : 1); }
template <class T> T floor(const T &a, const T &b) {
    assert(b != 0);
    return sign(a) * sign(b) > 0 ? abs(a) / abs(b)
                                 : -(abs(a) + abs(b) - 1) / abs(b);
}
template <class T> T ceil(const T &a, const T &b) {
    assert(b != 0);
    return sign(a) * sign(b) > 0 ? (abs(a) + abs(b) - 1) / abs(b)
                                 : -abs(a) / abs(b);
}
template <class T> bool tmin(T &a, T b) { return b < a ? a = b, true : false; }
template <class T> bool tmax(T &a, T b) { return b > a ? a = b, true : false; }
#if __cplusplus > 201103L
template <class T> auto min(const T &a) { return *min_element(rang(a)); }
template <class T> auto max(const T &a) { return *max_element(rang(a)); }
template <class T> auto sum(const T &a) {
    return accumulate(rang(a), (typename T::value_type)0);
}
template <> auto sum(const vector<int> &a) { return accumulate(rang(a), 0ll); }
template <> auto sum(const vector<string> &a) {
    return accumulate(rang(a), string());
}
template <class C, class V> auto find(C &c, const V &v) {
    return find(rang(c), v);
}
template <class C, class V> auto find(const C &c, const V &v) {
    return find(rang(c), v);
}
#endif
template <class T, class C = less<typename T::value_type>>
void sort(T &a, C c = C()) {
    sort(rang(a), c);
}
template <class T, class C> void sork(T &a, C c) {
    sort(a, [&](const typename T::value_type &x,
                const typename T::value_type &y) { return c(x) < c(y); });
}
template <class T> void reve(T &a) { reverse(rang(a)); }
template <class T> void uniq(T &a) {
    sort(a), a.erase(unique(rang(a)), end(a));
}
template <class T> void shuf(T &a) { shuffle(rang(a), rng); }
template <class T> void merg(T l, T m, T r) { inplace_merge(l, m, r); }
#if __cplusplus > 201103L
namespace lru_cache {
template <class F> struct dense_ {
    dense_(F function) : f(function) {}
    template <class V> auto build(int i) const {
        assert(i == l.size());
        return numeric_limits<V>::min();
    }
    template <class V, class T, class... K>
    auto build(int i, const T &t, const K &... k) const {
        auto u = build<V, K...>(i + 1, k...);
        return vector<decltype(u)>(l[i], u);
    }
    template <class M, class V> V &find(M &m) const { return m; }
    template <class M, class V, class T, class... K>
    V &find(M &m, const T &t, const K &... k) const {
        if (t < m.size())
            return find<decltype(m[t]), V, K...>(m[t], k...);
        return find<decltype(m[t]), V, K...>(m[e = false], k...);
    }
    template <class... K> auto operator()(const K &... k) const {
        typedef decltype(f(*this, k...)) V;
        typedef decltype(build<V, K...>(0, k...)) M;
        if (c) {
            if (p)
                delete (M *)p, p = 0;
            c = false;
        }
        if (!p)
            p = new M(move(build<V, K...>(0, k...)));
        e = true;
        auto &t = find<M, V, K...>(*((M *)p), k...);
        return e ? (t == numeric_limits<V>::min() ? t = f(*this, k...) : t)
                 : f(*this, k...);
    }
    F f;
    static vector<int> l;
    static void *p;
    static bool c, e;
};
template <class F> vector<int> dense_<F>::l;
template <class F> void *dense_<F>::p = 0;
template <class F> bool dense_<F>::c = false;
template <class F> bool dense_<F>::e = false;
template <class F> struct sparse_ {
    sparse_(F function) : f(function) {}
    template <class V> auto build() const { return V(); }
    template <class V, class T, class... K>
    auto build(const T &t, const K &... k) const {
        return unordered_map<T, decltype(build<V, K...>(k...))>();
    }
    template <class M, class V> V &find(M &m) const { return m; }
    template <class M, class V, class T, class... K>
    V &find(M &m, const T &t, const K &... k) const {
        auto i = m.find(t);
        if (i != m.end())
            return find<decltype(i->second), V, K...>(i->second, k...);
        e = false;
        return find<decltype(i->second), V, K...>(m[t], k...);
    }
    template <class... K> auto operator()(const K &... k) const {
        typedef decltype(f(*this, k...)) V;
        typedef decltype(build<V, K...>(k...)) M;
        if (c) {
            if (p)
                delete (M *)p, p = 0;
            c = false;
        }
        if (!p)
            p = new M(move(build<V, K...>(k...)));
        e = true;
        auto &t = find<M, V, K...>(*((M *)p), k...);
        return e ? t : t = f(*this, k...);
    }
    F f;
    static void *p;
    static bool c, e;
};
template <class F> void *sparse_<F>::p = 0;
template <class F> bool sparse_<F>::e = false;
template <class F> bool sparse_<F>::c = false;
struct dense {
    auto convert() {}
    template <class T, class... L>
    auto convert(const T &t, const L &... limits) {
        l.push_back(t);
        convert(limits...);
    }
    template <class... L> dense(const L &... limits) { convert(limits...); }
    template <class F> auto operator()(const F &f) const {
        dense_<F>::l = l, dense_<F>::c = true;
        return dense_<F>(f);
    }
    vector<int> l;
};
struct sparse {
    template <class F> auto operator()(const F &f) const {
        sparse_<F>::c = true;
        return sparse_<F>(f);
    }
};
} // namespace lru_cache
template <class T> auto vect(const T &v, int n) { return vector<T>(n, v); }
template <class T, class... D> auto vect(const T &v, int n, D... m) {
    return vector<decltype(vect(v, m...))>(n, vect(v, m...));
}
#endif
template <class F, class S> istream &operator>>(istream &s, pair<F, S> &a) {
    return s >> a.first >> a.second;
}
template <class F, class S>
ostream &operator<<(ostream &s, const pair<F, S> &a) {
    return s << a.first << " " << a.second;
}
template <class T> istream &operator>>(istream &s, vector<T> &a) {
    for (int i = -1; i < 0 && (!a.size() && (cin >> i, a.resize(i), 0), i = 0),
             i < int(a.size());)
        s >> a[i++];
    return s;
}
template <class T> ostream &operator<<(ostream &s, const vector<T> &a) {
    lv(i, a) cout << a[i] << (i + 1 == int(a.size()) ? "" : " ");
    return s;
}
template <class T> T in() {
    T a;
    cin >> a;
    return a;
}
template <class T> istream &in(T &a) { return cin >> a; }
template <class A, class... B> istream &in(A &a, B &... b) {
    return cin >> a, in(b...);
}
void ou() { cout << endl; }
template <class T> void ou(const T &a) { cout << a << endl; }
template <class A, class... B> void ou(const A &a, const B &... b) {
    cout << a << ' ', ou(b...);
}
bool yep(const bool &a) { return ou(a ? "yes" : "no"), a; }
bool Yep(const bool &a) { return ou(a ? "Yes" : "No"), a; }
bool YEP(const bool &a) { return ou(a ? "YES" : "NO"), a; }
template <class T> struct hash_safe {};
template <> struct hash_safe<int> {
    size_t operator()(unsigned long long a) const {
        static unsigned long long d =
            chrono::steady_clock::now().time_since_epoch().count();
        a += d + 0x9e3779b97f4a7c15, a = (a ^ (a >> 30)) * 0xbf58476d1ce4e5b9;
        a = (a ^ (a >> 27)) * 0x94d049bb133111eb;
        return a ^ (a >> 31);
    }
};
template <> struct hash_safe<long long> {
    size_t operator()(unsigned long long a) const {
        return hash_safe<int>()(a);
    }
};
template <class A, class B> struct hash_safe<pair<A, B>> {
    size_t operator()(pair<A, B> a) const {
        auto h = hash_safe<A>()(a.first), g = hash_safe<B>()(a.second);
        return (h >> 16 << 16) + (g >> 16);
    }
};
template <class T> using unordered_set_safe = unordered_set<T, hash_safe<T>>;
template <class T>
using unordered_multiset_safe = unordered_multiset<T, hash_safe<T>>;
template <class K, class V>
using unordered_map_safe = unordered_map<K, V, hash_safe<K>>;
template <class K, class V>
using unordered_multimap_safe = unordered_multimap<K, V, hash_safe<K>>;
#define unordered_set unordered_set_safe
#define unordered_multiset unordered_multiset_safe
#define unordered_map unordered_map_safe
#define unordered_multimap unordered_multimap_safe
template <class T, class C = less<T>>
using bbst =
    tree<T, null_type, C, rb_tree_tag, tree_order_statistics_node_update>;
template <class T, class C = greater<T>>
using heap = __gnu_pbds::priority_queue<T, C, pairing_heap_tag>;
using ll = long long;
using ld = long double;
using str = string;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
using vpii = vector<pii>;
using vpll = vector<pll>;
using vvi = vector<vi>;
namespace bfs_forest {
struct bfs_forest {
    vector<vector<int>> to;
    vector<bool> visited;
    vector<int> depth, component;
    int component_number;
    bfs_forest(int n)
        : to(n), visited(n), depth(n), component(n), component_number(0) {}
    void add_edge(int u, int v) {
        to.at(u).push_back(v), to.at(v).push_back(u);
    }
    void run_single(int s) {
        if (!visited[s]) {
            queue<int> q;
            q.push(s), visited[s] = true, component[s] = component_number++;
            while (q.size()) {
                int u = q.front();
                q.pop();
                for (int v : to[u])
                    if (!visited[v])
                        visited[v] = true, q.push(v), depth[v] = depth[u] + 1,
                        component[v] = component_number - 1;
            }
        }
    }
    void run() {
        for (int s = 0; s < int(to.size()); ++s)
            run_single(s);
    }
};
} // namespace bfs_forest
// The templates end here.
int main() {
    ios::sync_with_stdio(0);
    cout << setprecision(16) << fixed;
    cin.tie(0);
    int n, m;
    in(n, m);
    bfs_forest::bfs_forest a(n);
    while (m--) {
        int u, v;
        in(u, v);
        a.add_edge(u - 1, v - 1);
    }
    a.run();
    ou(a.component_number - 1);
    return 0;
}


Measured memory_bytes: 9316