#pragma region header
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define int long long
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define rev(i, n) for(int i = (int)(n - 1); i >= 0; i--)
#define rev1(i, n) for(int i = (int)(n); i > 0; i--)
#define pb push_back
#define all(v) (v).begin(), (v).end()
#define resort(v) sort((v).rbegin(), (v).rend())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vc vector<char>
#define vvc vector<vector<char>>
#define vb vector<bool>
#define vvb vector<vector<bool>>
using ll = long long;
using P = pair<int, int>;
/* ----------------よく使う数字や配列----------------- */
int dx[] = {1,0,-1,0};
int dy[] = {0,1,0,-1};
constexpr ll mod = 1e9+7;
constexpr int inf = INT32_MAX/2;
constexpr ll INF = LLONG_MAX/2;
constexpr long double eps = DBL_EPSILON;
constexpr long double pi = 3.141592653589793238462643383279;
/* ----------------------end----------------------- */

/* --------------------テンプレート------------------ */
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
/* ----------------------end----------------------- */

/* --------------------ライブラリ-------------------- */
ll fact(int i) {       //階乗
    if (i == 0) return 1;
    return (fact(i - 1)) * i % mod;
}
ll gcd(ll a, ll b) {        //最大公約数
    if(b == 0) return a;
    return gcd(b, a % b); 
}
ll lcm(ll a, ll b) {      //最小公倍数
    return a / gcd(a, b) * b ;
}
int keta(ll n) {        //桁数を求める
    if(n == 0) return 1;
    int count = 0;
    while(n != 0) {
        n /= 10;
        count++;
    }
    return count;
}
ll ketasum(ll n) {    //各桁の和
    ll sum = 0;
    while(n != 0) {
        sum += n % 10;
        n /= 10;
    }
    return sum;
}
bool isprime(int n) { //素数判定
    if(n==1)return false;
    if(n==2)return true;
    for(int i = 2;i<=sqrt(n);i++) {
        if(n%i==0) return false;
    }
    return true;
}
/* ----------------------end----------------------- */
#pragma endregion
struct UnionFind {
    vector<int> par, size;
    UnionFind(int x) {
        par.resize(x);
        size.resize(x, 1);
        for(int i = 0; i < x; i++) {
            par[i] = i;
        }
    }
    int find(int x) {
        if (par[x] == x)
            return x;
        return par[x] = find(par[x]);
    }
    bool same(int x, int y) {
        return find(x) == find(y);
    }
    int consize(int x) {
        return size[find(x)];
    }
    void unite(int x, int y) {
        x = find(x);
        y = find(y);
        if (x == y)
            return;
        if (size[x] < size[y]) {
            par[x] = y;
            size[y] += size[x];
        }
        else {
            par[y] = x;
            size[x] += size[y];
        }
    }
};
signed main() {
    int n,m;cin >> n >> m;
    UnionFind uf(n);
    rep(i, m) {
        int a,b;cin >> a >> b;
        a--,b--;
        uf.unite(a,b);
    }
    set<int> st;
    rep(i, n) st.insert(uf.find(i));
    cout << st.size()-1 << endl;
    return 0;
}

Measured memory_bytes: 8496