/*
 * じょえチャンネル
 * 高評価・チャンネル登録よろしくおねがいします！
 * https://www.youtube.com/channel/UCRXsI3FL_kvaVL9zoolBfbQ
 */

#include <algorithm>
#include <bitset>
#include <cassert>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <ctime>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string.h>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#include <atcoder/all>

//#pragma GCC target("avx2")
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")

//here!!!
// define int long long !!!!!

#define int long long

// define int long long !!!!!

//#define mod 1000000007ll
constexpr int mod = 998244353ll;

constexpr double PI = 3.141592653589793238462643383279;
//constexpr double eps = DBL_EPSILON;

typedef long long ll;

#ifdef int
#define inf (int)(3e18)
#else
#define inf (int)(5e8)
#endif

#define intt long long
#define itn long long
#define innt long long
#define P pair<long long, long long>

#define rep(i, n) for (int i = 0; i < n; i++)
#define REP(i, n) for (int i = 1; i <= n; i++)
#define rev_rep(i, n) for (int i = n - 1; i >= 0; i--)
#define REV_REP(i, n) for (int i = n; i >= 1; i--)

#define ALL(v) v.begin(), v.end()

#define smallpriority_queue(T) priority_queue<T, vector<T>, greater<T>>

using namespace std;

//Library
//モッドパウ
inline int modpow(int x, int y, int m = mod) {
    int res = 1;
    while (y) {
        if (y & 1) {
            res *= x;
            res %= m;
        }
        x = x * x % m;
        y /= 2;
    }
    return res;
}

int mypow(int x, int y) {
    int res = 1;
    while (y) {
        if (y % 2) {
            res *= x;
        }
        x = x * x;
        y /= 2;
    }
    return res;
}
//is the number (x) a prime number?
//bool prime(int x) {
//    if (!x || x == 1) {
//        return false;
//    }
//    for (int i = 2; i * i <= x; i++) {
//        if (!(x % i)) {
//            return false;
//        }
//    }
//    return true;
//}
bool prime(int x) {
    if (!x || x == 1) {
        return false;
    }
    if (x == 2) {
        return true;
    }
    if (!(x & 1)) {
        return false;
    }
    for (int i = 3; i * i <= x; i += 2) {
        if (!(x % i)) {
            return false;
        }
    }
    return true;
}

//saidai-kouyakusuu
inline int gcd(int x, int y) {
    if (!y) {
        return x;
    }
    return gcd(y, x % y);
}

//number of keta
int keta(int x) {
    int ans = 0;
    while (x) {
        x /= 10;
        ans++;
    }
    return ans;
}

//number of 2shinsuu keta
int bitketa(int x) {
    int ans = 0;
    while (x) {
        x >>= 1;
        ans++;
    }
    return ans;
}

//sum of keta
int ketasum(int x) {
    int ans = 0;
    while (x) {
        ans += x % 10;
        x /= 10;
    }
    return ans;
}

inline int lcm(int x, int y) {
    int ans = x / gcd(x, y) * y;
    return ans;
}
int twobeki(int x) {
    int ans = 0;
    while (1) {
        if (!(x & 1)) {
            ans++;
            x >>= 1;
        } else {
            break;
        }
    }
    return ans;
}

template <class T, class U>
inline bool chmax(T &lhs, const U &rhs) {
    if (lhs < rhs) {
        lhs = rhs;
        return 1;
    }
    return 0;
}
template <class T, class U>
inline bool chmin(T &lhs, const U &rhs) {
    if (lhs > rhs) {
        lhs = rhs;
        return 1;
    }
    return 0;
}
void Yes() {
    cout << "Yes" << endl;
}
void No() {
    cout << "No" << endl;
}
void YES() {
    cout << "YES" << endl;
}
void NO() {
    cout << "NO" << endl;
}

#define fin(i) scanf("%lld", &i)
#define fout(i) printf("%lld", i)
#define fendl printf("\n")

int kai(int x, int y) {
    int res = 1;
    for (int i = x - y + 1; i <= x; i++) {
        res *= i;
        res %= mod;
    }
    return res;
}

int comb(int x, int y) {
    if (y > x)
        return 0;
    //    cout<<kai(x, y)<<' '<<modpow(kai(y, y), mod - 2)<<endl;
    return kai(x, y) * modpow(kai(y, y), mod - 2) % mod;
}
//Library-End

//入出力高速化時にはoff！！！！
//#define vecin(v) for(int i=0;i<v.size();i++)scanf("%lld",&v[i]);
//#define vecout(v) {if(v.size())printf("%lld",v[0]);for(int i=1;i<(int)v.size();i++)printf(" %lld",v[i]);printf("\n");}

template <class T>
class SegTree {
    int n;
    vector<T> node;
    T def;
    function<T(T, T)> operation;
    function<T(T, T)> update;

public:
    SegTree(unsigned int _n, T _def, function<T(T, T)> _operation,
            function<T(T, T)> _update)
        : def(_def), operation(_operation), update(_update) {
        n = 1;
        while (n < _n) {
            n *= 2;
        }
        node = vector<T>(n * 2, def);
    }
    SegTree(vector<int> &initvec, function<T(T, T)> _operation,
            function<T(T, T)> _update)
        : operation(_operation), update(_update) {
        n = 1;
        while (n < initvec.size()) {
            n *= 2;
        }
        node = vector<T>(n * 2, def);
        for (int i = n; i < n + initvec.size(); i++) {
            node[i] = initvec[i - n];
        }
        for (int i = n - 1; i >= 1; i--) {
            node[i] = operation(node[i * 2], node[i * 2 + 1]);
        }
    }
    void change(int i, T x) {
        i += n;
        node[i] = update(node[i], x);
        while (i >= 1) {
            i >>= 1;
            node[i] = operation(node[i * 2], node[i * 2 + 1]);
        }
    }
    T query(int l, int r) {
        l += n;
        r += n;
        T rx = def, lx = def;
        while (l < r) {
            if (l & 1) {
                lx = operation(lx, node[l]);
                l++;
            }
            if (r & 1) {
                r--;
                rx = operation(node[r], rx);
            }
            l >>= 1;
            r >>= 1;
        }
        return operation(lx, rx);
    }
    T operator[](const int &x) {
        return node[x + n];
    }
    void fill(T x) {
        std::fill(ALL(node), x);
    }
    void print() {
        rep(i, n) std::cout << operator[](i) << " ";
        std::cout << std::endl;
    }
    T queryForALL() {
        return node[1];
    }
};

#define R_MIN ([](long long a, long long b) { return min(a, b); })
#define R_MAX ([](long long a, long long b) { return max(a, b); })
#define R_SUM ([](long long a, long long b) { return a + b; })

#define NORMAL_UPDATE ([](long long a, long long b) { return b; })
#define ADD_UPDATE ([](long long a, long long b) { return a + b; })
#define MINUS_UPDATE ([](long long a, long long b) { return a - b; }

class Union_Find {
    vector<int> par;
    vector<int> ookisa;

public:
    Union_Find(int size) {
        par = vector<int>(size);
        ookisa = vector<int>(size);
        for (int i = 0; i < size; i++) {
            par[i] = i;
            ookisa[i] = 1;
        }
    }

    int find(int x) {
        if (par[x] == x) {
            return x;
        }
        return par[x] = find(par[x]);
    }

    void unite(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y) {
            return;
        }
        if (ookisa[x] < ookisa[y]) {
            par[x] = y;
            ookisa[y] += ookisa[x];
            ookisa[x] = 0;
        } else {
            par[y] = x;
            ookisa[x] += ookisa[y];
            ookisa[y] = 0;
        }
    }
    int size(int i) {
        i = find(i);
        return ookisa[i];
    }
    bool same(int x, int y) {
        return find(x) == find(y);
    }
};

class BIT {
    vector<int> data;
    int size = 0;

public:
    BIT(int _size) {
        data = vector<int>(_size + 1);
        size = _size;
    }
    void add(int i, int x) {
        while (i <= size) {
            data[i] += x;
            i += i & -i;
        }
    }
    int sum(int i) {
        assert(i <= size);
        int s = 0;
        while (i > 0) {
            s += data[i];
            i -= i & -i;
        }
        return s;
    }
    int lower_bound(int x) {
        if (x <= 0) {
            return 0;
        } else {
            int i = 0;
            int r = 1;
            while (r < size)
                r = r << 1;
            for (int len = r; len > 0; len = len >> 1) {
                if (i + len < size && data[i + len] < x) {
                    x -= data[i + len];
                    i += len;
                }
            }
            return i + 1;
        }
    }
};

//Union-Find-End

int perm[2000005];
void init_perm() {
    perm[0] = 1;
    REP(i, 2000004) {
        perm[i] = perm[i - 1] * i % mod;
    }
}

int nCk(int x, int y) {
    if (y > x) {
        return 0;
    }
    if (x < 0) {
        return 0;
    }
    return perm[x] * modpow(perm[x - y], mod - 2) % mod * modpow(perm[y], mod - 2) % mod;
}

struct Dot {
    double x;
    double y;
    Dot(double _x = 0.0, double _y = 0.0) {
        x = _x;
        y = _y;
    }
};

struct Dot_i {
    int x;
    int y;
    Dot_i(int _x = 0, int _y = 0) {
        x = _x;
        y = _y;
    }
};

double kyori(pair<int, int> f, pair<int, int> s) {
    double ans = 0;
    double t = fabs(f.first - s.first);
    double y = fabs(f.second - s.second);
    ans = sqrt(t * t + y * y);
    return ans;
}

double kyori(Dot f, Dot s) {
    double ans = 0;
    double t = fabs(f.x - s.x);
    double y = fabs(f.y - s.y);
    ans = sqrt(t * t + y * y);
    return ans;
}

inline bool stringmax(string &x, string &y) {
    if (x.size() != y.size()) {
        return x.size() < y.size();
    }
    return x < y;
}

//vector<int>  RollingHash(string &s, string& t){
//    vector<int> ans;
//    if(s.size()<t.size())return ans;
//    __int128 h=0,hamod=0,ki=0,kim=0,hikaku=0,one=0;
//    one=1;
//    ki=1000000007ll;
//    hamod=(one<<61)-1;
//    kim=1;
//    rep(i,t.size()){
//        hikaku*=ki;
//        h*=ki;
//        kim*=ki;
//        hikaku+=t[i];
//        h+=s[i];
//        hikaku%=hamod;
//        h%=hamod;
//        kim%=hamod;
//    }
//    rep(i,(int)s.size()-(int)t.size()){
//        if (h==hikaku) {
//            ans.emplace_back(i);
//        }
//        h*=ki;
//        h%=hamod;
//        h+=s[i+(int)t.size()];
//        h%=hamod;
//        h+=hamod;
//        h-=s[i]*kim%hamod;
//        h%=hamod;
//    }
//    if(h==hikaku)ans.emplace_back((int)s.size()-(int)t.size());
//    return ans;
//}
struct edge {
    int to;
    int length;
    edge(int _to, int _length) {
        to = _to;
        length = _length;
    }
};
vector<int> djkstra(vector<vector<edge>> &road, int start) {
    vector<int> kyo(road.size(), inf);
    smallpriority_queue(P) q;
    q.push({0, start});
    kyo[start] = 0;
    while (q.size()) {
        int x = q.top().second;
        itn now = q.top().first;
        q.pop();
        if (kyo[x] < now) {
            continue;
        }
        for (auto &i : road[x]) {
            if (kyo[i.to] > now + i.length) {
                kyo[i.to] = now + i.length;
                q.push({kyo[i.to], i.to});
            }
        }
    }
    return kyo;
}

template <class T>
void change_to_unique(vector<T> &v) {
    std::sort(ALL(v));
    auto k = unique(ALL(v));
    if (k != v.end())
        v.erase(k, v.end());
}

double kodo_dosuu(double r) {
    return 180.0 * r / (double)PI;
}

double dosuu_kodo(double r) {
    return r * (double)PI / 180.0;
}

double kakudo(Dot a, Dot b1, Dot b2) {
    double size1 = kyori(a, b1), size2 = kyori(a, b2);
    double nai = (b1.x - a.x) * (b2.x - a.x) + (b1.y - a.y) * (b2.y - a.y);
    nai /= size1 * size2;
    return kodo_dosuu(acos(nai));
}

struct fraction {
    int shi;
    int bo;
    fraction(int bunshi, int bunbo) : bo(bunbo), shi(bunshi) {
        if (bunbo && bunshi) {
            int g = gcd(bo, shi);
            bo /= g;
            shi /= g;
        }
    };
    explicit inline operator double() const { return (double)shi / (double)bo; }
    explicit inline operator long double() const { return (long double)shi / (long double)bo; }
};
bool operator<(const fraction &b1, const fraction &b2) {
    return b1.shi * b2.bo < b2.shi * b1.bo;
}
bool operator>(const fraction &b1, const fraction &b2) {
    return b1.shi * b2.bo > b2.shi * b1.bo;
}

template <class T>
void vecout(vector<T> &v) {
    if (v.size()) {
        cout << v[0];
    }
    REP(i, (int)v.size() - 1) {
        cout << ' ' << v[i];
    }
    cout << endl;
}

inline int zeronCk(int x, int y) {
    return nCk(y + x - 1, x - 1);
}

inline int modinv(int x, int MOD = mod) {
    return modpow(x, MOD - 2);
}

#define endl "\n" //interactive の時に注意！！！
#define Endl "\n" //interactive の時に注意！！！
#define cinn cin
#define printd cout << fixed << setprecision(10)
#define rrep(i, f, s) for (int i = f; i < s; i++)
#define RREP(i, f, s) for (int i = f; i <= s; i++)
#define REV_RREP(i, f, s) for (int i = s; i >= f; i--)
#define hen_rep(i, l, step) for (int i = 0; i < l; i += step)
#define HEN_REP(i, l, step) for (int i = 1; i <= l; i += step)

int n, m, a[100004], b[100004], ans;
signed main() {
    ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cin >> n >> m;
    Union_Find uni(n + 10);
    rep(i, m) {
        cin >> a[i] >> b[i];
        uni.unite(a[i], b[i]);
    }
    REP(i, n) {
        if (!uni.same(1, i)) {
            ans++;
            uni.unite(1, i);
        }
    }
    cout << ans << endl;
}


Measured memory_bytes: 6412