///////////////////////////////////////////////////////////////////////////////

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

using namespace std;

///////////////////////////////////////////////////////////////////////////////

#define DEBUG 0

#define pb push_back
#define V vector
#define M unordered_map
#define S static

#define rep(i,n) for(ll i=0LL; i<n; ++i)
#define srep(i,s,n) for(ll i=s; i<n; ++i)
#define rrep(i,n) for(ll i=n-1LL; i>=0LL; --i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define CIN(x) do { \
        assert(!cin.eof()); \
        cin >> x; \
        assert(!cin.fail()); \
} while(0);
#if DEBUG
#define debug_print(...) _debug_print(__VA_ARGS__)
#define debug_printf(...) printf(__VA_ARGS__)
#define debug_print_time _debug_print_time
#else // DEBUG
#define debug_print(...)
#define debug_printf(...)
#define debug_print_time
#endif // DEBUG

typedef long long ll;
typedef unsigned long long ull;
typedef tuple<ll, ll> t2;
typedef tuple<ll, ll, ll> t3;
typedef tuple<ll, ll, ll, ll> t4;
typedef tuple<ll, ll, ll, ll, ll> t5;
template <typename T>
using priority_queue_incr = priority_queue<T, V<T>, greater<T> >;

///////////////////////////////////////////////////////////////////////////////

template <typename TT, typename T>
T get_m(M<TT, T> &m, TT k, T default_value)
{
    if (m.find(k) == m.end()) return m[k] = default_value;
    return m[k];
}

template <typename TT, typename T>
void incr_m(M<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

template <typename TT, typename T>
void incr_m(map<TT, T> &m, TT k)
{
    if (m.find(k) == m.end()) m[k] = 0;
    m[k]++;
}

void llin(ll &a)
{
    CIN(a);
}

void llinl1(V<ll> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a;
        CIN(a);
        v.push_back(a);
    }
}

void llinl2(V<t2> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b;
        CIN(a >> b);
        v.push_back(t2(a, b));
    }
}

void llinl3(V<t3> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c;
        CIN(a >> b >> c);
        v.push_back(t3(a, b, c));
    }
}

void llinl4(V<t4> &v, ll count)
{
    for (ll i = 0LL; i < count; ++i) {
        ll a, b, c, d;
        CIN(a >> b >> c >> d);
        v.push_back(t4(a, b, c, d));
    }
}

void llina(V<ll> &v, ll count)
{
    llinl1(v, count);
}

template <typename T>
void sort(V<T> &v)
{
    sort(v.begin(), v.end());
}

template <typename T>
void sort_reverse(V<T> &v)
{
    sort(v.begin(), v.end(), greater<T>());
}

template <typename T>
void _debug_print(T x)
{
    cout << x << endl;
}

void _debug_print(const t2 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    cout << "-- " << x1 << " -- " << x2 << endl;
}

void _debug_print(const t3 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    cout << "-- " << x1 << " -- " << x2 << " -- " << x3 << endl;
}

void _debug_print(const t4 &x)
{
    ll x1 = get<0>(x);
    ll x2 = get<1>(x);
    ll x3 = get<2>(x);
    ll x4 = get<3>(x);
    cout << "-- " << x1 << " -- " << x2
         << " -- " << x3 << " -- " << x4 << endl;
}

template <typename T>
void _debug_print(T xarray[], ll n)
{
    rep (i, n) _debug_print(xarray[i]);
}

template <typename T>
void _debug_print(const V<T> &xlist)
{
    for (auto x : xlist) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename T>
void _debug_print(const set<T> &xset)
{
    for (auto x : xset) {
        cout << "-- ";
        _debug_print(x);
    }
}

template <typename TT, typename T>
void _debug_print(const M<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

template <typename TT, typename T>
void _debug_print(const map<TT, T> &xlist)
{
    for (auto x : xlist) {
        TT k = x.first;
        T v = x.second;
        cout << "====" << endl;
        cout << "K=";
        _debug_print(k);
        cout << "V=";
        _debug_print(v);
    }
}

void _debug_print_time(const char *prefix)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
#if DEBUG
    struct tm *tm = localtime(&tv.tv_sec);
#endif
    debug_printf("-- %s %02d:%02d:%02d.%06ld\n",
                  prefix, tm->tm_hour, tm->tm_min, tm->tm_sec, tv.tv_usec);
}



struct UnionFind
{
    ull *parent, *count, *rank;

    UnionFind(ull n) {
        parent = new ull[n+1];
        count = new ull[n+1];
        rank = new ull[n+1];
        for (ull i = 0ULL; i < n+1; ++i) {
            parent[i] = i;
            count[i] = 1;
            rank[i] = 0;
        }
    }

    ~UnionFind() {
        delete rank;
        delete count;
        delete parent;
    }

    ull root(ull i) {
        if (parent[i] == i) return i;
        parent[i] = root(parent[i]);
        return parent[i];
    }

    void unite(ull i, ull j) {
        ull rooti = root(i);
        ull rootj = root(j);

        if (rooti == rootj) return;

        if (rank[rootj] < rank[rooti]) {
            parent[i] = parent[j] = parent[rootj] = rooti;
            count[rooti] += count[rootj];
        }
        else {
            parent[i] = parent[j] = parent[rooti] = rootj;
            count[rootj] += count[rooti];
            if (rank[rootj] == rank[rooti]) rank[rootj]++;
        }
    }

    bool same(ull i, ull j) {
        return root(i) == root(j);
    }
};

struct UnionFindM
{
    M<ull, ull> parent, count, rank;

    ull root(ull i) {
        ull parent_i = get_m(parent, i, i);
        if (parent_i == i) return i;
        return parent[i] = root(parent_i);
    }

    void unite(ull i, ull j) {
        ull rooti = root(i);
        ull rootj = root(j);

        if (rooti == rootj) return;

        if (get_m(rank, rootj, 0ULL) < get_m(rank, rooti, 0ULL)) {
            parent[i] = parent[j] = parent[rootj] = rooti;
            count[rooti] = get_m(count, rooti, 1ULL) + get_m(count, rootj, 1ULL);
        }
        else {
            parent[i] = parent[j] = parent[rooti] = rootj;
            count[rootj] = get_m(count, rootj, 1ULL) + get_m(count, rooti, 1ULL);
            if (get_m(rank, rootj, 0ULL) == get_m(rank, rooti, 0ULL)) rank[rootj]++;
        }
    }

    bool same(ull i, ull j) {
        return root(i) == root(j);
    }
};



///////////////////////////////////////////////////////////////////////////////

void _main();
int main()
{
    cout << setprecision(12);
#if !DEBUG
    ios::sync_with_stdio(false);
    cin.tie(0);
#endif

    _main();
    return 0;
}

void _main()
{
    ll n; llin(n);
    ll m; llin(m);
    V<t2> ablist; llinl2(ablist, m);
    S UnionFind uf(100005);
    for (auto ab :ablist) {
        ll a = get<0>(ab);
        ll b = get<1>(ab);
        uf.unite(a,b);
    }
    set<ll> roots;
    srep (v, 1, n+1) roots.insert(uf.root(v));
    cout << roots.size() - 1 << endl;

}

///////////////////////////////////////////////////////////////////////////////


Measured memory_bytes: 9492