Problem p03203 (C++)

Buggy submission (status: Runtime Error):
#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rfor(i, m, n) for (int i = (m); i >= (n); --i)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define range_it(a, l, r) (a).begin() + (l), (a).begin() + (r)

using namespace std;
using ll = long long;
using ld = long double;
using VB = vector<bool>;
using VVB = vector<VB>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using VS = vector<string>;
using VD = vector<ld>;
using PII = pair<int, int>;
using VP = vector<PII>;
using PLL = pair<ll, ll>;
using VPL = vector<PLL>;
template <class T> using PQ = priority_queue<T>;
template <class T> using PQS = priority_queue<T, vector<T>, greater<T>>;
constexpr int inf = (int)1e9;
constexpr ll inf_ll = (ll)1e18, MOD = 1000000007;
constexpr ld PI = M_PI, EPS = 1e-12;

// --- input --- //
#ifdef _WIN32
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#define fwrite_unlocked fwrite
#define fflush_unlocked _fflush_nolock
#endif
inline int gc() { return getchar_unlocked(); }
template <class T> inline void InputF(T &v) { cin >> v; }
inline void InputF(char &v) {
  while (isspace(v = gc()))
    ;
}
inline void InputF(bool &v) {
  char c;
  InputF(c);
  v = c != '0';
}
inline void InputF(string &v) {
  v.clear();
  char c;
  for (InputF(c); !isspace(c); c = gc())
    v += c;
}
inline void InputF(int &v) {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc())
    v = v * 10 + (c - '0');
  if (neg)
    v = -v;
}
inline void InputF(long long &v) {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc())
    v = v * 10 + (c - '0');
  if (neg)
    v = -v;
}
inline void InputF(double &v) {
  double dp = 1;
  bool neg = false, adp = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c) || c == '.'; c = gc()) {
    if (c == '.')
      adp = true;
    else if (adp)
      v += (c - '0') * (dp *= 0.1);
    else
      v = v * 10 + (c - '0');
  }
  if (neg)
    v = -v;
}
template <class T, class U> inline void InputF(pair<T, U> &v) {
  InputF(v.first);
  InputF(v.second);
}
template <class T> inline void InputF(vector<T> &v) {
  for (auto &e : v)
    InputF(e);
}
template <class T> inline T InputF() {
  T v;
  InputF(v);
  return v;
}
inline string GetLine() {
  string v;
  char c;
  for (InputF(c); c != '\n' && c != '\0'; c = gc())
    v += c;
  return v;
}
struct InputV {
  int n, m;
  InputV(int N) : n(N), m(0) {}
  InputV(pair<int, int> N) : n(N.first), m(N.second) {}
  template <class T> operator vector<T>() {
    vector<T> v(n);
    InputF(v);
    return v;
  }
  template <class T> operator vector<vector<T>>() {
    vector<vector<T>> v(n, vector<T>(m));
    InputF(v);
    return v;
  }
};
struct Input {
  template <class T> operator T() { return InputF<T>(); }
  int operator--(int) {
    int v;
    InputF(v);
    v--;
    return v;
  }
  InputV operator[](int n) { return InputV(n); }
  InputV operator[](pair<int, int> n) { return InputV(n); }
  void operator()() {}
  template <class H, class... T> void operator()(H &&h, T &&...t) {
    InputF(h);
    operator()(forward<T>(t)...);
  }
  template <class T> Input &operator,(T &&v) {
    InputF(v);
    return *this;
  }
} in;
#define input(T) InputF<T>()
#define INT input(int)
#define LL input(ll)
#define STR input(string)
#define inputs(T, ...)                                                         \
  T __VA_ARGS__;                                                               \
  in(__VA_ARGS__)
#define ini(...) inputs(int, __VA_ARGS__)
#define inl(...) inputs(ll, __VA_ARGS__)
#define ins(...) inputs(string, __VA_ARGS__)

// --- output --- //
struct BoolStr {
  const char *t, *f;
  BoolStr(const char *_t, const char *_f) : t(_t), f(_f) {}
} Yes("Yes", "No"), yes("yes", "no"), YES("YES", "NO"), Int("1", "0");
struct DivStr {
  const char *d, *l;
  DivStr(const char *_d, const char *_l) : d(_d), l(_l) {}
} spc(" ", "\n"), no_spc("", "\n"), end_line("\n", "\n"), comma(",", "\n"),
    no_endl(" ", "");
class Output {
  BoolStr B{Yes};
  DivStr D{spc};
  void p(int v) {
    if (v < 0)
      putchar_unlocked('-'), v = -v;
    char b[10];
    int i = 0;
    while (v)
      b[i++] = '0' + v % 10, v /= 10;
    if (!i)
      b[i++] = '0';
    while (i--)
      putchar_unlocked(b[i]);
  }
  void p(ll v) {
    if (v < 0)
      putchar_unlocked('-'), v = -v;
    char b[20];
    int i = 0;
    while (v)
      b[i++] = '0' + v % 10, v /= 10;
    if (!i)
      b[i++] = '0';
    while (i--)
      putchar_unlocked(b[i]);
  }
  void p(bool v) { p(v ? B.t : B.f); }
  void p(char v) { putchar_unlocked(v); }
  void p(const char *v) { fwrite_unlocked(v, 1, strlen(v), stdout); }
  void p(double v) { printf("%.20f", v); }
  void p(ld v) { printf("%.20Lf", v); }
  template <class T> void p(const T &v) { cout << v; }
  template <class T, class U> void p(const pair<T, U> &v) {
    p(v.first);
    p(D.d);
    p(v.second);
  }
  template <class T> void p(const vector<T> &v) {
    rep(i, sz(v)) {
      if (i)
        p(D.d);
      p(v[i]);
    }
  }
  template <class T> void p(const vector<vector<T>> &v) {
    rep(i, sz(v)) {
      if (i)
        p(D.l);
      p(v[i]);
    }
  }

public:
  Output &operator()() {
    p(D.l);
    return *this;
  }
  template <class H> Output &operator()(H &&h) {
    p(h);
    p(D.l);
    return *this;
  }
  template <class H, class... T> Output &operator()(H &&h, T &&...t) {
    p(h);
    p(D.d);
    return operator()(forward<T>(t)...);
  }
  template <class... T> void exit(T &&...t) {
    operator()(forward<T>(t)...);
    std::exit(EXIT_SUCCESS);
  }
  Output &flush() {
    fflush_unlocked(stdout);
    return *this;
  }
  Output &set(const BoolStr &b) {
    B = b;
    return *this;
  }
  Output &set(const DivStr &d) {
    D = d;
    return *this;
  }
  Output &set(const char *t, const char *f) {
    B = BoolStr(t, f);
    return *this;
  }
} out;

// --- step --- //
template <class T> struct Step {
  class It {
    T a, b, c;

  public:
    constexpr It() : a(T()), b(T()), c(T()) {}
    constexpr It(T _b, T _c, T _s) : a(_b), b(_c), c(_s) {}
    constexpr It &operator++() {
      --b;
      a += c;
      return *this;
    }
    constexpr It operator++(int) {
      It tmp = *this;
      --b;
      a += c;
      return tmp;
    }
    constexpr const T &operator*() const { return a; }
    constexpr const T *operator->() const { return &a; }
    constexpr bool operator==(const It &i) const { return b == i.b; }
    constexpr bool operator!=(const It &i) const { return !(b == i.b); }
    constexpr T start() const { return a; }
    constexpr T count() const { return b; }
    constexpr T step() const { return c; }
  };
  constexpr Step(T b, T c, T s) : be(b, c, s) {}
  constexpr It begin() const { return be; }
  constexpr It end() const { return en; }
  constexpr T start() const { return be.start(); }
  constexpr T count() const { return be.count(); }
  constexpr T step() const { return be.step(); }
  constexpr T sum() const {
    return start() * count() + step() * (count() * (count() - 1) / 2);
  }
  operator vector<T>() const { return as_vector(); }
  template <class F> void each(const F &f) const {
    for (T i : *this)
      f(i);
  }
  vector<T> as_vector() const {
    vector<T> res;
    res.reserve(count());
    each([&](T i) { res.push_back(i); });
    return res;
  }
  template <class F, class U = decay_t<result_of_t<F(T)>>>
  auto map(const F &f) const {
    vector<U> res;
    res.reserve(count());
    each([&](T i) { res.push_back(f(i)); });
    return res;
  }
  template <class F> vector<T> select(const F &f) const {
    vector<T> res;
    each([&](T i) {
      if (f(i))
        res.push_back(i);
    });
    return res;
  }
  template <class F> int count_if(const F &f) const {
    int res = 0;
    each([&](T i) {
      if (f(i))
        ++res;
    });
    return res;
  }
  template <class F> T find_if(const F &f) const {
    for (T i : *this)
      if (f(i))
        return i;
    return 0;
  }
  template <class F> auto max_by(const F &f) const {
    auto v = map(f);
    return *max_element(v.begin(), v.end());
  }
  template <class F> auto min_by(const F &f) const {
    auto v = map(f);
    return *min_element(v.begin(), v.end());
  }
  template <class F, class U = decay_t<result_of_t<F(T)>>>
  auto sum(const F &f) const {
    U res = 0;
    each([&](T i) { res += static_cast<U>(f(i)); });
    return res;
  }
  using value_type = T;
  using iterator = It;

private:
  It be, en;
};
template <class T> inline constexpr auto step(T a) { return Step<T>(0, a, 1); }
template <class T> inline constexpr auto step(T a, T b) {
  return Step<T>(a, b - a, 1);
}
template <class T> inline constexpr auto step(T a, T b, T c) {
  return Step<T>(a, a < b ? (b - a - 1) / c + 1 : 0, c);
}

// --- functions --- //
inline namespace {
template <class T> inline void Sort(T &a) { sort(all(a)); }
template <class T> inline void RSort(T &a) { sort(rall(a)); }
template <class T> inline T Sorted(T a) {
  Sort(a);
  return a;
}
template <class T> inline T RSorted(T a) {
  RSort(a);
  return a;
}
template <class T, class F> inline void Sort(T &a, const F &f) {
  sort(all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class F> inline void RSort(T &a, const F &f) {
  sort(rall(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T> inline void Reverse(T &a) { reverse(all(a)); }
template <class T> inline void Unique(T &a) {
  a.erase(unique(all(a)), a.end());
}
template <class T> inline void Uniq(T &a) {
  Sort(a);
  Unique(a);
}
template <class T> inline void Rotate(T &a, int left) {
  rotate(a.begin(), a.begin() + left, a.end());
}
template <class T> inline T Reversed(T a) {
  Reverse(a);
  return a;
}
template <class T> inline T Uniqued(T a) {
  Unique(a);
  return a;
}
template <class T> inline T Uniqed(T a) {
  Uniq(a);
  return a;
}
template <class T> inline T Rotated(T a, int left) {
  Rotate(a, left);
  return a;
}
template <class T> inline auto Max(const T &a) { return *max_element(all(a)); }
template <class T> inline auto Min(const T &a) { return *min_element(all(a)); }
template <class T> inline int MaxPos(const T &a) {
  return max_element(all(a)) - a.begin();
}
template <class T> inline int MinPos(const T &a) {
  return min_element(all(a)) - a.begin();
}
template <class T, class F> inline auto Max(const T &a, const F &f) {
  return *max_element(
      all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class F> inline auto Min(const T &a, const F &f) {
  return *min_element(
      all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class U> inline int Count(const T &a, const U &v) {
  return count(all(a), v);
}
template <class T, class F> inline int CountIf(const T &a, const F &f) {
  return count_if(all(a), f);
}
template <class T, class U> inline int Find(const T &a, const U &v) {
  return find(all(a), v) - a.begin();
}
template <class T, class F> inline int FindIf(const T &a, const F &f) {
  return find_if(all(a), f) - a.begin();
}
template <class T, class U = typename T::value_type> inline U Sum(const T &a) {
  return accumulate(all(a), U());
}
template <class T, class F> inline auto Sum(const T &v, const F &f) {
  return accumulate(next(v.begin()), v.end(), f(v.front()),
                    [&](auto a, auto b) { return a + f(b); });
}
template <class T, class U> inline int Lower(const T &a, const U &v) {
  return lower_bound(all(a), v) - a.begin();
}
template <class T, class U> inline int Upper(const T &a, const U &v) {
  return upper_bound(all(a), v) - a.begin();
}
template <class T, class F> inline void RemoveIf(T &a, const F &f) {
  a.erase(remove_if(all(a), f), a.end());
}
template <class F> inline auto Vector(size_t size, const F &f) {
  vector<decay_t<result_of_t<F(size_t)>>> res(size);
  for (size_t i = 0; i < size; ++i)
    res[i] = f(i);
  return res;
}
template <class T> inline auto Grid(size_t h, size_t w, const T &v = T()) {
  return vector<vector<T>>(h, vector<T>(w, v));
}
template <class T> inline auto Slice(const T &v, size_t i, size_t len) {
  return i < v.size() ? T(v.begin() + i, v.begin() + min(i + len, v.size()))
                      : T();
}
template <class T, class F> inline auto Each(const T &v, const F &f) {
  for (auto &i : v)
    f(i);
}
template <class T, class F> inline auto Select(const T &v, const F &f) {
  T res;
  for (const auto &e : v)
    if (f(e))
      res.push_back(e);
  return res;
}
template <class T, class F> inline auto Map(const T &v, const F &f) {
  vector<decay_t<result_of_t<F(typename T::value_type)>>> res(v.size());
  size_t i = 0;
  for (const auto &e : v)
    res[i++] = f(e);
  return res;
}
template <class T, class F> inline auto MapIndex(const T &v, const F &f) {
  vector<decay_t<result_of_t<F(size_t, typename T::value_type)>>> res(v.size());
  size_t i = 0;
  for (auto it = v.begin(); it != v.end(); ++it, ++i)
    res[i] = f(i, *it);
  return res;
}
template <class T, class F> inline auto TrueIndex(const T &v, const F &f) {
  vector<size_t> res;
  for (size_t i = 0; i < v.size(); ++i)
    if (f(v[i]))
      res.push_back(i);
  return res;
}
inline string operator*(string s, size_t n) {
  string ret;
  for (size_t i = 0; i < n; ++i)
    ret += s;
  return ret;
}
template <class T> inline T Ceil(T n, T m) { return (n + m - 1) / m; }
template <class T> inline T Ceil2(T n, T m) { return Ceil(n, m) * m; }
template <class T> inline T Tri(T n) {
  return (n & 1) ? (n + 1) / 2 * n : n / 2 * (n + 1);
}
template <class T> inline T nC2(T n) {
  return (n & 1) ? (n - 1) / 2 * n : n / 2 * (n - 1);
}
template <class T> inline T Mid(const T &l, const T &r) {
  return l + (r - l) / 2;
}
inline int pop_count(int n) { return bitset<32>(n).count(); }
inline int pop_count(ll n) { return bitset<64>(n).count(); }
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool inRange(const T &v, const T &min, const T &max) {
  return min <= v && v < max;
}
template <class T = ll> inline T BIT(int b) { return T(1) << b; }
template <class T> inline T Gcd(T n, T m) { return m ? Gcd(m, n % m) : n; }
template <class T> inline T Lcm(T n, T m) { return n / Gcd(n, m) * m; }
template <class T, class U = typename T::value_type> inline U Gcdv(const T &v) {
  return accumulate(next(v.begin()), v.end(), U(*v.begin()), Gcd<U>);
}
template <class T, class U = typename T::value_type> inline U Lcmv(const T &v) {
  return accumulate(next(v.begin()), v.end(), U(*v.begin()), Lcm<U>);
}
template <class T> inline T Pow(T a, T n) {
  T r = 1;
  while (n > 0) {
    if (n & 1)
      r *= a;
    a *= a;
    n /= 2;
  }
  return r;
}
template <class T> inline T Powmod(T a, T n, T m = MOD) {
  T r = 1;
  while (n > 0) {
    if (n & 1)
      r = r * a % m, n--;
    else
      a = a * a % m, n /= 2;
  }
  return r;
}
} // namespace

// --- dump --- //
#if __has_include("dump.hpp")
#include "dump.hpp"
#else
#define dump(...) ((void)0)
#endif

// ---------------------------------------------------------------- //

int main() {
  ini(h, w, n);
  vector<set<int>> s(w);
  rep(i, n) {
    int x = in--, y = in--;
    s[x].insert(y);
  }

  int bottom = 0;
  FOR(x, 1, w) {
    if (s[x].size() && *s[x].begin() <= bottom) {
      out.exit(x);
    }
    if (!s[x].count(bottom + 1)) {
      bottom++;
    }
  }
  out(w);
}

Fixed submission:
#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define FOR(i, m, n) for (int i = (m); i < (n); ++i)
#define rrep(i, n) for (int i = (n)-1; i >= 0; --i)
#define rfor(i, m, n) for (int i = (m); i >= (n); --i)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define range_it(a, l, r) (a).begin() + (l), (a).begin() + (r)

using namespace std;
using ll = long long;
using ld = long double;
using VB = vector<bool>;
using VVB = vector<VB>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;
using VS = vector<string>;
using VD = vector<ld>;
using PII = pair<int, int>;
using VP = vector<PII>;
using PLL = pair<ll, ll>;
using VPL = vector<PLL>;
template <class T> using PQ = priority_queue<T>;
template <class T> using PQS = priority_queue<T, vector<T>, greater<T>>;
constexpr int inf = (int)1e9;
constexpr ll inf_ll = (ll)1e18, MOD = 1000000007;
constexpr ld PI = M_PI, EPS = 1e-12;

// --- input --- //
#ifdef _WIN32
#define getchar_unlocked _getchar_nolock
#define putchar_unlocked _putchar_nolock
#define fwrite_unlocked fwrite
#define fflush_unlocked _fflush_nolock
#endif
inline int gc() { return getchar_unlocked(); }
template <class T> inline void InputF(T &v) { cin >> v; }
inline void InputF(char &v) {
  while (isspace(v = gc()))
    ;
}
inline void InputF(bool &v) {
  char c;
  InputF(c);
  v = c != '0';
}
inline void InputF(string &v) {
  v.clear();
  char c;
  for (InputF(c); !isspace(c); c = gc())
    v += c;
}
inline void InputF(int &v) {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc())
    v = v * 10 + (c - '0');
  if (neg)
    v = -v;
}
inline void InputF(long long &v) {
  bool neg = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c); c = gc())
    v = v * 10 + (c - '0');
  if (neg)
    v = -v;
}
inline void InputF(double &v) {
  double dp = 1;
  bool neg = false, adp = false;
  v = 0;
  char c;
  InputF(c);
  if (c == '-') {
    neg = true;
    c = gc();
  }
  for (; isdigit(c) || c == '.'; c = gc()) {
    if (c == '.')
      adp = true;
    else if (adp)
      v += (c - '0') * (dp *= 0.1);
    else
      v = v * 10 + (c - '0');
  }
  if (neg)
    v = -v;
}
template <class T, class U> inline void InputF(pair<T, U> &v) {
  InputF(v.first);
  InputF(v.second);
}
template <class T> inline void InputF(vector<T> &v) {
  for (auto &e : v)
    InputF(e);
}
template <class T> inline T InputF() {
  T v;
  InputF(v);
  return v;
}
inline string GetLine() {
  string v;
  char c;
  for (InputF(c); c != '\n' && c != '\0'; c = gc())
    v += c;
  return v;
}
struct InputV {
  int n, m;
  InputV(int N) : n(N), m(0) {}
  InputV(pair<int, int> N) : n(N.first), m(N.second) {}
  template <class T> operator vector<T>() {
    vector<T> v(n);
    InputF(v);
    return v;
  }
  template <class T> operator vector<vector<T>>() {
    vector<vector<T>> v(n, vector<T>(m));
    InputF(v);
    return v;
  }
};
struct Input {
  template <class T> operator T() { return InputF<T>(); }
  int operator--(int) {
    int v;
    InputF(v);
    v--;
    return v;
  }
  InputV operator[](int n) { return InputV(n); }
  InputV operator[](pair<int, int> n) { return InputV(n); }
  void operator()() {}
  template <class H, class... T> void operator()(H &&h, T &&...t) {
    InputF(h);
    operator()(forward<T>(t)...);
  }
  template <class T> Input &operator,(T &&v) {
    InputF(v);
    return *this;
  }
} in;
#define input(T) InputF<T>()
#define INT input(int)
#define LL input(ll)
#define STR input(string)
#define inputs(T, ...)                                                         \
  T __VA_ARGS__;                                                               \
  in(__VA_ARGS__)
#define ini(...) inputs(int, __VA_ARGS__)
#define inl(...) inputs(ll, __VA_ARGS__)
#define ins(...) inputs(string, __VA_ARGS__)

// --- output --- //
struct BoolStr {
  const char *t, *f;
  BoolStr(const char *_t, const char *_f) : t(_t), f(_f) {}
} Yes("Yes", "No"), yes("yes", "no"), YES("YES", "NO"), Int("1", "0");
struct DivStr {
  const char *d, *l;
  DivStr(const char *_d, const char *_l) : d(_d), l(_l) {}
} spc(" ", "\n"), no_spc("", "\n"), end_line("\n", "\n"), comma(",", "\n"),
    no_endl(" ", "");
class Output {
  BoolStr B{Yes};
  DivStr D{spc};
  void p(int v) {
    if (v < 0)
      putchar_unlocked('-'), v = -v;
    char b[10];
    int i = 0;
    while (v)
      b[i++] = '0' + v % 10, v /= 10;
    if (!i)
      b[i++] = '0';
    while (i--)
      putchar_unlocked(b[i]);
  }
  void p(ll v) {
    if (v < 0)
      putchar_unlocked('-'), v = -v;
    char b[20];
    int i = 0;
    while (v)
      b[i++] = '0' + v % 10, v /= 10;
    if (!i)
      b[i++] = '0';
    while (i--)
      putchar_unlocked(b[i]);
  }
  void p(bool v) { p(v ? B.t : B.f); }
  void p(char v) { putchar_unlocked(v); }
  void p(const char *v) { fwrite_unlocked(v, 1, strlen(v), stdout); }
  void p(double v) { printf("%.20f", v); }
  void p(ld v) { printf("%.20Lf", v); }
  template <class T> void p(const T &v) { cout << v; }
  template <class T, class U> void p(const pair<T, U> &v) {
    p(v.first);
    p(D.d);
    p(v.second);
  }
  template <class T> void p(const vector<T> &v) {
    rep(i, sz(v)) {
      if (i)
        p(D.d);
      p(v[i]);
    }
  }
  template <class T> void p(const vector<vector<T>> &v) {
    rep(i, sz(v)) {
      if (i)
        p(D.l);
      p(v[i]);
    }
  }

public:
  Output &operator()() {
    p(D.l);
    return *this;
  }
  template <class H> Output &operator()(H &&h) {
    p(h);
    p(D.l);
    return *this;
  }
  template <class H, class... T> Output &operator()(H &&h, T &&...t) {
    p(h);
    p(D.d);
    return operator()(forward<T>(t)...);
  }
  template <class... T> void exit(T &&...t) {
    operator()(forward<T>(t)...);
    std::exit(EXIT_SUCCESS);
  }
  Output &flush() {
    fflush_unlocked(stdout);
    return *this;
  }
  Output &set(const BoolStr &b) {
    B = b;
    return *this;
  }
  Output &set(const DivStr &d) {
    D = d;
    return *this;
  }
  Output &set(const char *t, const char *f) {
    B = BoolStr(t, f);
    return *this;
  }
} out;

// --- step --- //
template <class T> struct Step {
  class It {
    T a, b, c;

  public:
    constexpr It() : a(T()), b(T()), c(T()) {}
    constexpr It(T _b, T _c, T _s) : a(_b), b(_c), c(_s) {}
    constexpr It &operator++() {
      --b;
      a += c;
      return *this;
    }
    constexpr It operator++(int) {
      It tmp = *this;
      --b;
      a += c;
      return tmp;
    }
    constexpr const T &operator*() const { return a; }
    constexpr const T *operator->() const { return &a; }
    constexpr bool operator==(const It &i) const { return b == i.b; }
    constexpr bool operator!=(const It &i) const { return !(b == i.b); }
    constexpr T start() const { return a; }
    constexpr T count() const { return b; }
    constexpr T step() const { return c; }
  };
  constexpr Step(T b, T c, T s) : be(b, c, s) {}
  constexpr It begin() const { return be; }
  constexpr It end() const { return en; }
  constexpr T start() const { return be.start(); }
  constexpr T count() const { return be.count(); }
  constexpr T step() const { return be.step(); }
  constexpr T sum() const {
    return start() * count() + step() * (count() * (count() - 1) / 2);
  }
  operator vector<T>() const { return as_vector(); }
  template <class F> void each(const F &f) const {
    for (T i : *this)
      f(i);
  }
  vector<T> as_vector() const {
    vector<T> res;
    res.reserve(count());
    each([&](T i) { res.push_back(i); });
    return res;
  }
  template <class F, class U = decay_t<result_of_t<F(T)>>>
  auto map(const F &f) const {
    vector<U> res;
    res.reserve(count());
    each([&](T i) { res.push_back(f(i)); });
    return res;
  }
  template <class F> vector<T> select(const F &f) const {
    vector<T> res;
    each([&](T i) {
      if (f(i))
        res.push_back(i);
    });
    return res;
  }
  template <class F> int count_if(const F &f) const {
    int res = 0;
    each([&](T i) {
      if (f(i))
        ++res;
    });
    return res;
  }
  template <class F> T find_if(const F &f) const {
    for (T i : *this)
      if (f(i))
        return i;
    return 0;
  }
  template <class F> auto max_by(const F &f) const {
    auto v = map(f);
    return *max_element(v.begin(), v.end());
  }
  template <class F> auto min_by(const F &f) const {
    auto v = map(f);
    return *min_element(v.begin(), v.end());
  }
  template <class F, class U = decay_t<result_of_t<F(T)>>>
  auto sum(const F &f) const {
    U res = 0;
    each([&](T i) { res += static_cast<U>(f(i)); });
    return res;
  }
  using value_type = T;
  using iterator = It;

private:
  It be, en;
};
template <class T> inline constexpr auto step(T a) { return Step<T>(0, a, 1); }
template <class T> inline constexpr auto step(T a, T b) {
  return Step<T>(a, b - a, 1);
}
template <class T> inline constexpr auto step(T a, T b, T c) {
  return Step<T>(a, a < b ? (b - a - 1) / c + 1 : 0, c);
}

// --- functions --- //
inline namespace {
template <class T> inline void Sort(T &a) { sort(all(a)); }
template <class T> inline void RSort(T &a) { sort(rall(a)); }
template <class T> inline T Sorted(T a) {
  Sort(a);
  return a;
}
template <class T> inline T RSorted(T a) {
  RSort(a);
  return a;
}
template <class T, class F> inline void Sort(T &a, const F &f) {
  sort(all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class F> inline void RSort(T &a, const F &f) {
  sort(rall(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T> inline void Reverse(T &a) { reverse(all(a)); }
template <class T> inline void Unique(T &a) {
  a.erase(unique(all(a)), a.end());
}
template <class T> inline void Uniq(T &a) {
  Sort(a);
  Unique(a);
}
template <class T> inline void Rotate(T &a, int left) {
  rotate(a.begin(), a.begin() + left, a.end());
}
template <class T> inline T Reversed(T a) {
  Reverse(a);
  return a;
}
template <class T> inline T Uniqued(T a) {
  Unique(a);
  return a;
}
template <class T> inline T Uniqed(T a) {
  Uniq(a);
  return a;
}
template <class T> inline T Rotated(T a, int left) {
  Rotate(a, left);
  return a;
}
template <class T> inline auto Max(const T &a) { return *max_element(all(a)); }
template <class T> inline auto Min(const T &a) { return *min_element(all(a)); }
template <class T> inline int MaxPos(const T &a) {
  return max_element(all(a)) - a.begin();
}
template <class T> inline int MinPos(const T &a) {
  return min_element(all(a)) - a.begin();
}
template <class T, class F> inline auto Max(const T &a, const F &f) {
  return *max_element(
      all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class F> inline auto Min(const T &a, const F &f) {
  return *min_element(
      all(a), [&](const auto &x, const auto &y) { return f(x) < f(y); });
}
template <class T, class U> inline int Count(const T &a, const U &v) {
  return count(all(a), v);
}
template <class T, class F> inline int CountIf(const T &a, const F &f) {
  return count_if(all(a), f);
}
template <class T, class U> inline int Find(const T &a, const U &v) {
  return find(all(a), v) - a.begin();
}
template <class T, class F> inline int FindIf(const T &a, const F &f) {
  return find_if(all(a), f) - a.begin();
}
template <class T, class U = typename T::value_type> inline U Sum(const T &a) {
  return accumulate(all(a), U());
}
template <class T, class F> inline auto Sum(const T &v, const F &f) {
  return accumulate(next(v.begin()), v.end(), f(v.front()),
                    [&](auto a, auto b) { return a + f(b); });
}
template <class T, class U> inline int Lower(const T &a, const U &v) {
  return lower_bound(all(a), v) - a.begin();
}
template <class T, class U> inline int Upper(const T &a, const U &v) {
  return upper_bound(all(a), v) - a.begin();
}
template <class T, class F> inline void RemoveIf(T &a, const F &f) {
  a.erase(remove_if(all(a), f), a.end());
}
template <class F> inline auto Vector(size_t size, const F &f) {
  vector<decay_t<result_of_t<F(size_t)>>> res(size);
  for (size_t i = 0; i < size; ++i)
    res[i] = f(i);
  return res;
}
template <class T> inline auto Grid(size_t h, size_t w, const T &v = T()) {
  return vector<vector<T>>(h, vector<T>(w, v));
}
template <class T> inline auto Slice(const T &v, size_t i, size_t len) {
  return i < v.size() ? T(v.begin() + i, v.begin() + min(i + len, v.size()))
                      : T();
}
template <class T, class F> inline auto Each(const T &v, const F &f) {
  for (auto &i : v)
    f(i);
}
template <class T, class F> inline auto Select(const T &v, const F &f) {
  T res;
  for (const auto &e : v)
    if (f(e))
      res.push_back(e);
  return res;
}
template <class T, class F> inline auto Map(const T &v, const F &f) {
  vector<decay_t<result_of_t<F(typename T::value_type)>>> res(v.size());
  size_t i = 0;
  for (const auto &e : v)
    res[i++] = f(e);
  return res;
}
template <class T, class F> inline auto MapIndex(const T &v, const F &f) {
  vector<decay_t<result_of_t<F(size_t, typename T::value_type)>>> res(v.size());
  size_t i = 0;
  for (auto it = v.begin(); it != v.end(); ++it, ++i)
    res[i] = f(i, *it);
  return res;
}
template <class T, class F> inline auto TrueIndex(const T &v, const F &f) {
  vector<size_t> res;
  for (size_t i = 0; i < v.size(); ++i)
    if (f(v[i]))
      res.push_back(i);
  return res;
}
inline string operator*(string s, size_t n) {
  string ret;
  for (size_t i = 0; i < n; ++i)
    ret += s;
  return ret;
}
template <class T> inline T Ceil(T n, T m) { return (n + m - 1) / m; }
template <class T> inline T Ceil2(T n, T m) { return Ceil(n, m) * m; }
template <class T> inline T Tri(T n) {
  return (n & 1) ? (n + 1) / 2 * n : n / 2 * (n + 1);
}
template <class T> inline T nC2(T n) {
  return (n & 1) ? (n - 1) / 2 * n : n / 2 * (n - 1);
}
template <class T> inline T Mid(const T &l, const T &r) {
  return l + (r - l) / 2;
}
inline int pop_count(int n) { return bitset<32>(n).count(); }
inline int pop_count(ll n) { return bitset<64>(n).count(); }
template <class T> inline bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> inline bool inRange(const T &v, const T &min, const T &max) {
  return min <= v && v < max;
}
template <class T = ll> inline T BIT(int b) { return T(1) << b; }
template <class T> inline T Gcd(T n, T m) { return m ? Gcd(m, n % m) : n; }
template <class T> inline T Lcm(T n, T m) { return n / Gcd(n, m) * m; }
template <class T, class U = typename T::value_type> inline U Gcdv(const T &v) {
  return accumulate(next(v.begin()), v.end(), U(*v.begin()), Gcd<U>);
}
template <class T, class U = typename T::value_type> inline U Lcmv(const T &v) {
  return accumulate(next(v.begin()), v.end(), U(*v.begin()), Lcm<U>);
}
template <class T> inline T Pow(T a, T n) {
  T r = 1;
  while (n > 0) {
    if (n & 1)
      r *= a;
    a *= a;
    n /= 2;
  }
  return r;
}
template <class T> inline T Powmod(T a, T n, T m = MOD) {
  T r = 1;
  while (n > 0) {
    if (n & 1)
      r = r * a % m, n--;
    else
      a = a * a % m, n /= 2;
  }
  return r;
}
} // namespace

// --- dump --- //
#if __has_include("dump.hpp")
#include "dump.hpp"
#else
#define dump(...) ((void)0)
#endif

// ---------------------------------------------------------------- //

int main() {
  ini(w, h, n);
  vector<set<int>> s(w);
  rep(i, n) {
    int x = in--, y = in--;
    s[x].insert(y);
  }

  int bottom = 0;
  FOR(x, 1, w) {
    if (s[x].size() && *s[x].begin() <= bottom) {
      out.exit(x);
    }
    if (!s[x].count(bottom + 1)) {
      bottom++;
    }
  }
  out(w);
}