#pragma region kyopro_template
#define Nyaan_template
#include <immintrin.h>
#include <bits/stdc++.h>
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define each(x, v) for (auto &x : v)
#define all(v) (v).begin(), (v).end()
#define sz(v) ((int)(v).size())
#define mem(a, val) memset(a, val, sizeof(a))
#define ini(...)   \
  int __VA_ARGS__; \
  in(__VA_ARGS__)
#define inl(...)         \
  long long __VA_ARGS__; \
  in(__VA_ARGS__)
#define ins(...)      \
  string __VA_ARGS__; \
  in(__VA_ARGS__)
#define inc(...)    \
  char __VA_ARGS__; \
  in(__VA_ARGS__)
#define in2(s, t)                           \
  for (int i = 0; i < (int)s.size(); i++) { \
    in(s[i], t[i]);                         \
  }
#define in3(s, t, u)                        \
  for (int i = 0; i < (int)s.size(); i++) { \
    in(s[i], t[i], u[i]);                   \
  }
#define in4(s, t, u, v)                     \
  for (int i = 0; i < (int)s.size(); i++) { \
    in(s[i], t[i], u[i], v[i]);             \
  }
#define rep(i, N) for (long long i = 0; i < (long long)(N); i++)
#define repr(i, N) for (long long i = (long long)(N)-1; i >= 0; i--)
#define rep1(i, N) for (long long i = 1; i <= (long long)(N); i++)
#define repr1(i, N) for (long long i = (N); (long long)(i) > 0; i--)
#define reg(i, a, b) for (long long i = (a); i < (b); i++)
#define die(...)      \
  do {                \
    out(__VA_ARGS__); \
    return;           \
  } while (0)
using namespace std;
using ll = long long;
template <class T>
using V = vector<T>;
using vi = vector<int>;
using vl = vector<long long>;
using vvi = vector<vector<int>>;
using vd = V<double>;
using vs = V<string>;
using vvl = vector<vector<long long>>;
using P = pair<long long, long long>;
using vp = vector<P>;
using pii = pair<int, int>;
using vpi = vector<pair<int, int>>;
constexpr int inf = 1001001001;
constexpr long long infLL = (1LL << 61) - 1;
template <typename T, typename U>
inline bool amin(T &x, U y) {
  return (y < x) ? (x = y, true) : false;
}
template <typename T, typename U>
inline bool amax(T &x, U y) {
  return (x < y) ? (x = y, true) : false;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  os << p.first << " " << p.second;
  return os;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T, U> &p) {
  is >> p.first >> p.second;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  int s = (int)v.size();
  for (int i = 0; i < s; i++) os << (i ? " " : "") << v[i];
  return os;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (auto &x : v) is >> x;
  return is;
}
void in() {}
template <typename T, class... U>
void in(T &t, U &... u) {
  cin >> t;
  in(u...);
}
void out() { cout << "\n"; }
template <typename T, class... U>
void out(const T &t, const U &... u) {
  cout << t;
  if (sizeof...(u)) cout << " ";
  out(u...);
}

#ifdef NyaanDebug
#define trc(...)                   \
  do {                             \
    cerr << #__VA_ARGS__ << " = "; \
    dbg_out(__VA_ARGS__);          \
  } while (0)
#define trca(v, N)       \
  do {                   \
    cerr << #v << " = "; \
    array_out(v, N);     \
  } while (0)
#define trcc(v)                             \
  do {                                      \
    cerr << #v << " = {";                   \
    each(x, v) { cerr << " " << x << ","; } \
    cerr << "}" << endl;                    \
  } while (0)
template <typename T>
void _cout(const T &c) {
  cerr << c;
}
void _cout(const int &c) {
  if (c == 1001001001)
    cerr << "inf";
  else if (c == -1001001001)
    cerr << "-inf";
  else
    cerr << c;
}
void _cout(const unsigned int &c) {
  if (c == 1001001001)
    cerr << "inf";
  else
    cerr << c;
}
void _cout(const long long &c) {
  if (c == 1001001001 || c == (1LL << 61) - 1)
    cerr << "inf";
  else if (c == -1001001001 || c == -((1LL << 61) - 1))
    cerr << "-inf";
  else
    cerr << c;
}
void _cout(const unsigned long long &c) {
  if (c == 1001001001 || c == (1LL << 61) - 1)
    cerr << "inf";
  else
    cerr << c;
}
template <typename T, typename U>
void _cout(const pair<T, U> &p) {
  cerr << "{ ";
  _cout(p.fi);
  cerr << ", ";
  _cout(p.se);
  cerr << " } ";
}
template <typename T>
void _cout(const vector<T> &v) {
  int s = v.size();
  cerr << "{ ";
  for (int i = 0; i < s; i++) {
    cerr << (i ? ", " : "");
    _cout(v[i]);
  }
  cerr << " } ";
}
template <typename T>
void _cout(const vector<vector<T>> &v) {
  cerr << "[ ";
  for (const auto &x : v) {
    cerr << endl;
    _cout(x);
    cerr << ", ";
  }
  cerr << endl << " ] ";
}
void dbg_out() { cerr << endl; }
template <typename T, class... U>
void dbg_out(const T &t, const U &... u) {
  _cout(t);
  if (sizeof...(u)) cerr << ", ";
  dbg_out(u...);
}
template <typename T>
void array_out(const T &v, int s) {
  cerr << "{ ";
  for (int i = 0; i < s; i++) {
    cerr << (i ? ", " : "");
    _cout(v[i]);
  }
  cerr << " } " << endl;
}
template <typename T>
void array_out(const T &v, int H, int W) {
  cerr << "[ ";
  for (int i = 0; i < H; i++) {
    cerr << (i ? ", " : "");
    array_out(v[i], W);
  }
  cerr << " ] " << endl;
}
#else
#define trc(...)
#define trca(...)
#define trcc(...)
#endif

inline int popcnt(unsigned long long a) { return __builtin_popcountll(a); }
inline int lsb(unsigned long long a) { return __builtin_ctzll(a); }
inline int msb(unsigned long long a) { return 63 - __builtin_clzll(a); }
template <typename T>
inline int getbit(T a, int i) {
  return (a >> i) & 1;
}
template <typename T>
inline void setbit(T &a, int i) {
  a |= (1LL << i);
}
template <typename T>
inline void delbit(T &a, int i) {
  a &= ~(1LL << i);
}
template <typename T>
int lb(const vector<T> &v, const T &a) {
  return lower_bound(begin(v), end(v), a) - begin(v);
}
template <typename T>
int ub(const vector<T> &v, const T &a) {
  return upper_bound(begin(v), end(v), a) - begin(v);
}
template <typename T>
int btw(T a, T x, T b) {
  return a <= x && x < b;
}
template <typename T, typename U>
T ceil(T a, U b) {
  return (a + b - 1) / b;
}
constexpr long long TEN(int n) {
  long long ret = 1, x = 10;
  while (n) {
    if (n & 1) ret *= x;
    x *= x;
    n >>= 1;
  }
  return ret;
}
template <typename T>
vector<T> mkrui(const vector<T> &v) {
  vector<T> ret(v.size() + 1);
  for (int i = 0; i < int(v.size()); i++) ret[i + 1] = ret[i] + v[i];
  return ret;
};
template <typename T>
vector<T> mkuni(const vector<T> &v) {
  vector<T> ret(v);
  sort(ret.begin(), ret.end());
  ret.erase(unique(ret.begin(), ret.end()), ret.end());
  return ret;
}
template <typename F>
vector<int> mkord(int N, F f) {
  vector<int> ord(N);
  iota(begin(ord), end(ord), 0);
  sort(begin(ord), end(ord), f);
  return ord;
}
template <typename T = int>
vector<T> mkiota(int N) {
  vector<T> ret(N);
  iota(begin(ret), end(ret), 0);
  return ret;
}
template <typename T>
vector<int> mkinv(vector<T> &v) {
  vector<int> inv(v.size());
  for (int i = 0; i < (int)v.size(); i++) inv[v[i]] = i;
  return inv;
}

struct IoSetupNya {
  IoSetupNya() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(15);
    cerr << fixed << setprecision(7);
  }
} iosetupnya;

void solve();
int main() { solve(); }

#pragma endregion
using namespace std;

template <typename T>
struct edge {
  int src, to;
  T cost;

  edge(int to, T cost) : src(-1), to(to), cost(cost) {}
  edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

  edge &operator=(const int &x) {
    to = x;
    return *this;
  }

  operator int() const { return to; }
};
template <typename T>
using Edges = vector<edge<T>>;
template <typename T>
using WeightedGraph = vector<Edges<T>>;
using UnweightedGraph = vector<vector<int>>;

// Input of (Unweighted) Graph
UnweightedGraph graph(int N, int M = -1, bool is_directed = false,
                      bool is_1origin = true) {
  UnweightedGraph g(N);
  if (M == -1) M = N - 1;
  for (int _ = 0; _ < M; _++) {
    int x, y;
    cin >> x >> y;
    if (is_1origin) x--, y--;
    g[x].push_back(y);
    if (!is_directed) g[y].push_back(x);
  }
  return g;
}

// Input of Weighted Graph
template <typename T>
WeightedGraph<T> wgraph(int N, int M = -1, bool is_directed = false,
                        bool is_1origin = true) {
  WeightedGraph<T> g(N);
  if (M == -1) M = N - 1;
  for (int _ = 0; _ < M; _++) {
    int x, y;
    cin >> x >> y;
    T c;
    cin >> c;
    if (is_1origin) x--, y--;
    g[x].eb(x, y, c);
    if (!is_directed) g[y].eb(y, x, c);
  }
  return g;
}

// Input of Edges
template <typename T>
Edges<T> esgraph(int N, int M, int is_weighted = true, bool is_1origin = true) {
  Edges<T> es;
  for (int _ = 0; _ < M; _++) {
    int x, y;
    cin >> x >> y;
    T c;
    if (is_weighted)
      cin >> c;
    else
      c = 1;
    if (is_1origin) x--, y--;
    es.emplace_back(x, y, c);
  }
  return es;
}

// Input of Adjacency Matrix
template <typename T>
vector<vector<T>> adjgraph(int N, int M, T INF, int is_weighted = true,
                           bool is_directed = false, bool is_1origin = true) {
  vector<vector<T>> d(N, vector<T>(N, INF));
  for (int _ = 0; _ < M; _++) {
    int x, y;
    cin >> x >> y;
    T c;
    if (is_weighted)
      cin >> c;
    else
      c = 1;
    if (is_1origin) x--, y--;
    d[x][y] = c;
    if (!is_directed) d[y][x] = c;
  }
  return d;
}
using namespace std;

template <uint32_t mod>
struct LazyMontgomeryModInt {
  using mint = LazyMontgomeryModInt;
  using i32 = int32_t;
  using u32 = uint32_t;
  using u64 = uint64_t;

  static constexpr u32 get_r() {
    u32 ret = mod;
    for (i32 i = 0; i < 4; ++i) ret *= 2 - mod * ret;
    return ret;
  }

  static constexpr u32 r = get_r();
  static constexpr u32 n2 = -u64(mod) % mod;
  static_assert(r * mod == 1, "invalid, r * mod != 1");
  static_assert(mod < (1 << 30), "invalid, mod >= 2 ^ 30");
  static_assert((mod & 1) == 1, "invalid, mod % 2 == 0");

  u32 a;

  constexpr LazyMontgomeryModInt() : a(0) {}
  constexpr LazyMontgomeryModInt(const int64_t &b)
      : a(reduce(u64(b % mod + mod) * n2)){};

  static constexpr u32 reduce(const u64 &b) {
    return (b + u64(u32(b) * u32(-r)) * mod) >> 32;
  }

  constexpr mint &operator+=(const mint &b) {
    if (i32(a += b.a - 2 * mod) < 0) a += 2 * mod;
    return *this;
  }

  constexpr mint &operator-=(const mint &b) {
    if (i32(a -= b.a) < 0) a += 2 * mod;
    return *this;
  }

  constexpr mint &operator*=(const mint &b) {
    a = reduce(u64(a) * b.a);
    return *this;
  }

  constexpr mint &operator/=(const mint &b) {
    *this *= b.inverse();
    return *this;
  }

  constexpr mint operator+(const mint &b) const { return mint(*this) += b; }
  constexpr mint operator-(const mint &b) const { return mint(*this) -= b; }
  constexpr mint operator*(const mint &b) const { return mint(*this) *= b; }
  constexpr mint operator/(const mint &b) const { return mint(*this) /= b; }
  constexpr bool operator==(const mint &b) const {
    return (a >= mod ? a - mod : a) == (b.a >= mod ? b.a - mod : b.a);
  }
  constexpr bool operator!=(const mint &b) const {
    return (a >= mod ? a - mod : a) != (b.a >= mod ? b.a - mod : b.a);
  }
  constexpr mint operator-() const { return mint() - mint(*this); }

  constexpr mint pow(u64 n) const {
    mint ret(1), mul(*this);
    while (n > 0) {
      if (n & 1) ret *= mul;
      mul *= mul;
      n >>= 1;
    }
    return ret;
  }
  
  constexpr mint inverse() const { return pow(mod - 2); }

  friend ostream &operator<<(ostream &os, const mint &b) {
    return os << b.get();
  }

  friend istream &operator>>(istream &is, mint &b) {
    int64_t t;
    is >> t;
    b = LazyMontgomeryModInt<mod>(t);
    return (is);
  }
  
  constexpr u32 get() const {
    u32 ret = reduce(a);
    return ret >= mod ? ret - mod : ret;
  }

  static constexpr u32 get_mod() { return mod; }
};
template <uint32_t MOD>
using Mint = LazyMontgomeryModInt<MOD>;
using namespace std;

template <typename T>
struct Binomial {
  vector<T> fac_, finv_, inv_;
  Binomial(int MAX) : fac_(MAX + 10), finv_(MAX + 10), inv_(MAX + 10) {
    MAX += 9;
    fac_[0] = finv_[0] = inv_[0] = 1;
    for (int i = 1; i <= MAX; i++) fac_[i] = fac_[i - 1] * i;
    finv_[MAX] = fac_[MAX].inverse();
    for (int i = MAX - 1; i > 0; i--) finv_[i] = finv_[i + 1] * (i + 1);
    for (int i = 1; i <= MAX; i++) inv_[i] = finv_[i] * fac_[i - 1];
  }

  inline T fac(int i) const { return fac_[i]; }
  inline T finv(int i) const { return finv_[i]; }
  inline T inv(int i) const { return inv_[i]; }

  T C(int n, int r) const {
    if (n < r || r < 0) return T(0);
    return fac_[n] * finv_[n - r] * finv_[r];
  }

  T C_naive(int n, int r) const {
    if (n < r || r < 0) return T(0);
    T ret = 1;
    for (T i = 1; i <= r; i += T(1)) {
      ret *= n--;
      ret *= i.inverse();
    }
    return ret;
  }

  T P(int n, int r) const {
    if (n < r || r < 0) return T(0);
    return fac_[n] * finv_[n - r];
  }

  T H(int n, int r) const {
    if (n < 0 || r < 0) return (0);
    return r == 0 ? 1 : C(n + r - 1, r);
  }
};



using namespace std;

struct UnionFind {
  vector<int> data;
  UnionFind(int N) : data(N, -1) {}

  int find(int k) { return data[k] < 0 ? k : data[k] = find(data[k]); }

  int unite(int x, int y) {
    if ((x = find(x)) == (y = find(y))) return false;
    if (data[x] > data[y]) swap(x, y);
    data[x] += data[y];
    data[y] = x;
    return true;
  }

  int size(int k) { return -data[find(k)]; }

  int same(int x, int y) { return find(x) == find(y); }
};

void solve() {
  ini(N,M);
  UnionFind uf(N);
  int n = N - 1;
  rep(_,M){
    ini(u,v);
    --u,--v;
    if(uf.unite(u,v))--n;
  }  
  out(n);
}

Measured memory_bytes: 3728