#line 2 "header.hpp"

//%snippet.set('header')%
//%snippet.fold()%
#ifndef HEADER_H
#define HEADER_H

// template version 2.0
using namespace std;
#include <bits/stdc++.h>

// varibable settings
const long long INF = 1e18;
template <class T> constexpr T inf = numeric_limits<T>::max() / 2.1;

#define _overload3(_1, _2, _3, name, ...) name
#define _rep(i, n) repi(i, 0, n)
#define repi(i, a, b) for (ll i = (ll)(a); i < (ll)(b); ++i)
#define rep(...) _overload3(__VA_ARGS__, repi, _rep, )(__VA_ARGS__)
#define _rrep(i, n) rrepi(i, 0, n)
#define rrepi(i, a, b) for (ll i = (ll)((b)-1); i >= (ll)(a); --i)
#define r_rep(...) _overload3(__VA_ARGS__, rrepi, _rrep, )(__VA_ARGS__)
#define each(i, a) for (auto &&i : a)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define pb(a) push_back(a)
#define mp(a, b) make_pair(a, b)
#define mt(...) make_tuple(__VA_ARGS__)
#define ub upper_bound
#define lb lower_bound
#define lpos(A, x) (lower_bound(all(A), x) - A.begin())
#define upos(A, x) (upper_bound(all(A), x) - A.begin())
template <class T, class U> inline void chmax(T &a, const U &b) { if ((a) < (b)) (a) = (b); }
template <class T, class U> inline void chmin(T &a, const U &b) { if ((a) > (b)) (a) = (b); }
template <typename X, typename T> auto make_table(X x, T a) { return vector<T>(x, a); }
template <typename X, typename Y, typename Z, typename... Zs> auto make_table(X x, Y y, Z z, Zs... zs) { auto cont = make_table(y, z, zs...); return vector<decltype(cont)>(x, cont); }

#define cdiv(a, b) (((a) + (b)-1) / (b))
#define is_in(x, a, b) ((a) <= (x) && (x) < (b))
#define uni(x) sort(all(x)); x.erase(unique(all(x)), x.end())
#define slice(l, r) substr(l, r - l)

typedef long long ll;
typedef long double ld;
using vl = vector<ll>;
using vvl = vector<vl>;
using pll = pair<ll, ll>;

template <typename T>
using PQ = priority_queue<T, vector<T>, greater<T>>;
void check_input() { assert(cin.eof() == 0); int tmp; cin >> tmp; assert(cin.eof() == 1); }

#if defined(PCM) || defined(LOCAL)
#else
#define dump(...) ;
#define dump_1d(...) ;
#define dump_2d(...) ;
#define cerrendl ;
#endif

#endif /* HEADER_H */
//%snippet.end()%
#line 2 "solve.cpp"
template<class T=ll> using vec = vector<T>;
struct Fast { Fast() { std::cin.tie(0); ios::sync_with_stdio(false); } } fast;

// {{{ include acl-library
// #include <atcoder/fenwicktree>
// #include <atcoder/segtree>
// #include <atcoder/lazysegtree>
// #include <atcoder/string>
// #include <atcoder/math>
// #include <atcoder/convolution>
// #include <atcoder/modint>
// #include <atcoder/dsu>
// #include <atcoder/maxflow>
// #include <atcoder/mincostflow>
// #include <atcoder/scc>
// #include <atcoder/twosat>
// using namespace atcoder;
// }}}

// snippet:UnionFind {{{
struct UnionFind {
    vector<int> par;   // par[x]: parent of x. if root, -size.
    int gcount;         // count of groups

    UnionFind() {}
    UnionFind(int _n) : par(_n, -1), gcount(_n) {}
    bool merge(int x, int y) { 
        x = root(x);
        y = root(y);
        if (x != y) {
            if (par[y] < par[x]) swap(x, y);
            par[x] += par[y];
            par[y] = x;
            gcount--;
        }
        return x != y;
    } 
    int root(int x) {
        if (is_root(x)){
            return x;
        }
        else{
            return par[x] = root(par[x]);  // 経路圧縮
            // return root(par[x]);         // 経路圧縮なし
        }
    }
    bool is_root(int x) { return par[x] < 0; }
    bool same(int x, int y) { return root(x) == root(y); }
    int size(int x) { return -par[root(x)]; }

#if defined(PCM) || defined(LOCAL)  // {{{
    friend ostream& operator<<(ostream& os, UnionFind& uf) {
        map<int, vector<int>> group;
        rep(i, sz(uf.par)) { group[uf.root(i)].pb(i); }
        os << endl;
        each(g, group) { os << g << endl; }
        return os;
    }
#endif  // }}}
};
// snippet:UnionFind }}}

int solve() {
    ll n,m;cin>>n>>m;

    UnionFind uf(n);
    rep(i, m){
        int u,v;cin>>u>>v;
        u--;v--;
        uf.merge(u, v);
    }
    cout << uf.gcount - 1<< endl;

    return 0; 
}


int main(){/*{{{*/
    solve();
    check_input();
    return 0;
}/*}}}*/


Measured memory_bytes: 3708