#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl '\n'
//#define int long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF 2147483647
#define LLINF 9223372036854775807LL
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define BIT(n) (1LL << (n))
#define TOUPPER(s) transform(s.begin(), s.end(), s.begin(), ::toupper)
#define TOLOWER(s) transform(s.begin(), s.end(), s.begin(), ::tolower)
#define FILLZERO(s, n)                      \
    do {                                    \
        ostringstream os;                   \
        os << setw(n) << setfill('0') << s; \
        s = os.str();                       \
    } while (0);
#define UNIQUE(v) v.erase(unique(v.begin(), v.end()), v.end()) //sortしてからつかうこと
#define INIT                               \
    std::cin.tie(nullptr);                 \
    std::ios_base::sync_with_stdio(false); \
    cout << std::fixed << std::setprecision(10);

using ll = long long;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using pi = pair<int, int>;
template <typename T>
using Pq = priority_queue<T>;
template <typename T>
using pQ = priority_queue<T, vector<T>, greater<T>>;

template <typename T>
vector<T> make_vec(size_t a, T n) { return vector<T>(a, n); }
template <typename T, typename... Ts>
auto make_vec(size_t a, Ts... ts) { return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...)); }
template <class T>
inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec)
        is >> x;
    return is;
}

constexpr ll MOD = 1e9 + 7;
//constexpr ll MOD = 998244353;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

struct UnionFind {
    vector<int> rank, parent, num;
    UnionFind(){};
    UnionFind(int size) : rank(size, 0), parent(size, 0), num(size, 0) {
        for (int i = 0; i < size; i++)
            makeset(i);
    }
    void makeset(int x) {
        parent[x] = x;
        rank[x] = x;
        num[x] = 1;
    }
    int size(int x) {
        return num[findset(x)];
    }
    bool same(int x, int y) {
        return findset(x) == findset(y);
    }
    int findset(int x) {
        if (x != parent[x]) {
            parent[x] = findset(parent[x]);
        }
        return parent[x];
    }
    void unite(int x, int y) {
        if (!same(x, y))
            link(findset(x), findset(y));
    }
    void link(int x, int y) {
        if (rank[x] > rank[y]) {
            parent[y] = x;
            num[x] += num[y];
        } else {
            parent[x] = y;
            num[y] += num[x];
            if (rank[x] == rank[y])
                rank[y]++;
        }
    }
};

signed main() {
    INIT;
    cin >> N >> M;
    UnionFind uf(N);
    REP(i, M) {
        cin >> A >> B;
        A--, B--;
        uf.unite(A, B);
    }
    map<int, int> mp;
    REP(i, N) {
        mp[uf.findset(i)]++;
    }
    cout << mp.size() - 1 << endl;
}

Measured memory_bytes: 8176