/*
Author- Aasav Badera
Time- Mon Sep 28 09:36:26 2020
*/
#include<bits/stdc++.h> 
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp> 
#pragma GCC optimize("-O3")
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma") 
using namespace std;
using namespace __gnu_pbds;
#ifdef LOCAL  
#include <sys/resource.h>
#define meminc             rlimit rlim;if (getrlimit(RLIMIT_STACK, &rlim)) return 1;rlim.rlim_cur = 268435456;if (setrlimit(RLIMIT_STACK, &rlim)) return 2;
#else
#define meminc      
#endif
#ifdef LOCAL
#define dbg(args...)       { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
#else
#define dbg(args...)
#endif
struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};
void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
    cerr << *it << " = " << a << endl;
    err(++it, args...);
}
#define ll                 int
#define vi                 vector<int >
#define ordered_set        tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
// -------------------Standard Traversal Moves---------------------
// vi fx = {1 ,-1 ,0, 0}, fy = {0, 0, -1, 1};
// vi fx = {2, -2, 2, -2,  1, -1, 1, -1}, fy = {1, 1, -1, -1, 2, 2, -2, -2};
// vi fx = {1, 1, 1, -1, -1 , -1, 0, 0}, fy = {1, -1, 0, 1, -1, 0, 1, -1};
// ----------------------------------------------------------------

#define rep(i, a, b)       for(int  i=a;i<b;i++)
#define all(a)             (a).begin(),(a).end()
#define sz(x)              (int )x.size()
#define yes                cout << "YES" << endl
#define no                 cout << "NO" << endl
#define endl               '\n' 
// const int hell = (int)998244353;
const int hell = (int)1e9 + 7;
const long long int inf = 3223372036854775807;
const double PI = 3.14159265;
const int N = (int) 1e6 + 5;
int n, m, k, a[N];


class dsu{
public:
    int _n;
    int seg;
    vector<int> root, sz;
    dsu(int _f) : _n(_f)
    {
        seg = _n;
        sz.resize(_n + 3);
        root.resize(_n + 3);
        rep(i, 0, _n + 3)
        {
            sz[i] = 1;
            root[i] = i;
        }
    }
    
    int find_set(int _v)
    {
        while(_v != root[_v]){
            root[_v] = root[root[_v]];
            _v = root[_v];
        }
        return _v;
    }
    void merge(int _a, int _b)
    {
        _a = find_set(_a);
        _b = find_set(_b);
        if(_a != _b)
        {
            if(sz[_a] < sz[_b])swap(_a, _b);
            root[_b] = _a;
            seg--;
            sz[_a] += sz[_b];
        }
    }
};
int32_t main()
{
    meminc;
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cerr.precision(10);
    cout.precision(25);
    cout << fixed;
    #ifdef LOCAL
    // for getting input from input.txt
    freopen("input.txt", "r", stdin);
    // for writing output to output.txt
    freopen("output.txt", "w", stdout);
    #endif
    int tests = 1;
    // cin >> tests;
    rep(test, 1, tests+1)
    {
        cin >> n;
        cin >> m;
        dsu graph(n);
        rep(i ,0 ,m)
        {
        	int u, v;
        	cin >> u >> v;
        	graph.merge(u, v);
        }
        cout << graph.seg - 1 << endl;
    }
    #ifdef LOCAL
        // cerr << "Time elapsed: " << 1.0 * clock() / CLOCKS_PER_SEC << " s.\n";
    #endif
    return 0;
}

Measured memory_bytes: 3916