#pragma once
#undef _GLIBCXX_DEBUG

#include "bits/stdc++.h"
#include "ext/pb_ds/assoc_container.hpp"
#include "ext/pb_ds/tree_policy.hpp"
#include "ext/rope"

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;

void trace(int x)                         { cerr << x; }
void trace(long x)                        { cerr << x; }
void trace(long long x)                   { cerr << x; }
void trace(unsigned x)                    { cerr << x; }
void trace(unsigned long x)               { cerr << x; }
void trace(unsigned long long x)          { cerr << x; }
void trace(float x)                       { cerr << x; }
void trace(double x)                      { cerr << x; }
void trace(long double x)                 { cerr << x; }
void trace(char x)                        { cerr << '\'' << x << '\''; }
void trace(const char *x)                 { cerr << '\"' << x << '\"'; }
void trace(const string &x)               { cerr << '\"' << x << '\"'; }
void trace(bool x)                        { cerr << (x ? "true" : "false"); }

template <typename A, typename B>                         void trace(const pair <A, B> &x)         { cerr << '{'; trace(x.first); cerr << ','; trace(x.second); cerr << '}'; }
template <typename A, typename B, typename C>             void trace(const tuple <A, B, C> &x)     { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << '}'; }
template <typename A, typename B, typename C, typename D> void trace(const tuple <A, B, C, D> &x)  { cerr << '{'; trace(get <0> (x)); cerr << ','; trace(get <1> (x)); cerr << ','; trace(get <2> (x)); cerr << ','; trace(get <3> (x)); cerr << '}'; }
template <typename T>                                     void trace(const T &x)                   { int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? "," : ""), trace(i); cerr << "}"; }
template <size_t N>                                       void trace(bitset <N> v)                 { cerr << '{'; for (size_t i = 0; i < N; i++) cerr << static_cast <char> ('0' + v[i]); cerr << '}'; }
void _print()                                                                                      { cerr << "]\n"; }
template <typename T, typename... V>                      void _print(T t, V... v)                 { trace(t); if (sizeof...(v)) cerr << ", "; _print(v...); }

#ifndef ONLINE_JUDGE
      #define debug(x...) cerr << "LINE(" << __LINE__ << ") -> [" << #x << "] = ["; _print(x)
#else
      #define debug(x...) 0
#endif // ONLINE_JUDGE

template <typename T> using oset                       = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename T1, typename T2> using omap         = tree <T1, T2, less <T1>, rb_tree_tag, tree_order_statistics_node_update>;

template <class T> bool MIN(T &a, T b)                 { return a > b ? (a = b, true) : false; }
template <class T> bool MAX(T &a, T b)                 { return a < b ? (a = b, true) : false; }

template <typename T> T binaryExpo(T a, T p)           { if (p == 0) { return 1LL; } if (p == 1) { return a; } if (p & 1) { return a * binaryExpo(a, p - 1); } T ret = binaryExpo(a, p / 2); return ret * ret; }
template <typename T> T bigMod(T a, T p, T m)          { if (p == 0) { return 1LL % m; } if (p == 1) { return a % m; } if (p & 1) { return (a % m * bigMod(a, p - 1, m)) % m; } T ret = bigMod(a, p / 2, m) % m; return (ret * ret) % m; }
template <typename T> T modInv(T a, T m)               { return bigMod(a, m - 2, m); }

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

//#define ll              long long int
#define ull             unsigned long long int
#define ld              long double
#define endl            '\n'
#define pii             pair <int, int>
#define pll             pair <ll, ll>
#define mk              make_pair
#define ff              first
#define ss              second
#define all(a)          (a).begin(), (a).end()
#define rall(a)         (a).rbegin(), (a).rend()
#define eb              emplace_back
#define pb              push_back
#define pf              push_front
#define allUpper(a)     transform(all(a), a.begin(), :: toupper)
#define allLower(a)     transform(all(a), a.begin(), :: tolower)
#define memo(a, b)      memset(a, b, sizeof a)
#define sz(a)           (int)(a.size())
#define ook             order_of_key             // The number of items in a set that are strictly smaller than k
#define fbo             find_by_order            // It returns an iterator to the i'th largest element

#define int             long long int

const int maxn = 3e5 + 5;
const int logn = 19;
const long long MOD = 1e9 + 7;
const long long INF = 1e18;

vector < vector <int> > graph;
bool vis[maxn];

void dfs(int u) {
      vis[u] = 1;
      for (int v : graph[u]) {
            if (vis[v]) {
                  continue;
            }
            dfs(v);
      }
}

signed main()
{
      ios_base::sync_with_stdio(false);
      cin.tie(nullptr); cout.tie(nullptr);
      cout.precision(12);

      bool FILEIO = 1;
      if (FILEIO and fopen("in.txt", "r")) {
            freopen("in.txt", "r", stdin);
      }

      int n, m;
      cin >> n >> m;
      graph.resize(n + 1);
      for (int e = 1; e <= m; e++) {
            int u, v;
            cin >> u >> v;
            graph[u].push_back(v);
            graph[v].push_back(u);
      }
      int ans = 0;
      for (int i = 1; i <= n; i++) {
            if (vis[i] == 0) {
                  ++ans;
                  dfs(i);
            }
      }
      cout << ans - 1 << endl;
}




Measured memory_bytes: 10020