Problem p03203 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const ll dx[9] = {0, -1, 0, 1, 1, -1, 1, -1, 0};
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;
}
int main() {
  ll h, w, n;
  cin >> h >> w >> n;
  vpl v(n);
  rep(i, n) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    v[i] = {a, b};
  }
  sort(all(v));
  vl dp(w);
  dp[0] = 1;
  ll now = 0, see = 0;
  rep(i, h) {
    ll z = now;
    while (z < n && v[z].fi == i) {
      if (dp[v[z].se])
        cout << i << endl, exit(0);
      z++;
    }
    now = z;
    if (!binary_search(all(v), make_pair(i + 1, see + 1))) {
      see++;
      dp[see]++;
    }
    // rep(j,w)cout << dp[j];cout << endl;
  }
  cout << h << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (ll i = 0; i < n; i++)
#define repl(i, l, r) for (ll i = (l); i < (r); i++)
#define per(i, n) for (ll i = n - 1; i >= 0; i--)
#define perl(i, r, l) for (ll i = r - 1; i >= l; i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x, vector<x>, greater<x>>
#define all(x) (x).begin(), (x).end()
#define CST(x) cout << fixed << setprecision(x)
#define vtpl(x, y, z) vector<tuple<x, y, z>>
#define rev(x) reverse(x);
using ll = long long;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using pl = pair<ll, ll>;
using vpl = vector<pl>;
using vvpl = vector<vpl>;
const ll MOD = 1000000007;
const ll MOD9 = 998244353;
const int inf = 1e9 + 10;
const ll INF = 4e18;
const ll dy[9] = {1, 0, -1, 0, 1, 1, -1, -1, 0};
const ll dx[9] = {0, -1, 0, 1, 1, -1, 1, -1, 0};
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;
}
int main() {
  ll h, w, n;
  cin >> h >> w >> n;
  vpl v(n);
  rep(i, n) {
    ll a, b;
    cin >> a >> b;
    a--;
    b--;
    v[i] = {a, b};
  }
  sort(all(v));
  vl dp(w);
  dp[0] = 1;
  ll now = 0, see = 0;
  rep(i, h) {
    ll z = now;
    while (z < n && v[z].fi == i) {
      if (dp[v[z].se])
        cout << i << endl, exit(0);
      z++;
    }
    now = z;
    if (!binary_search(all(v), make_pair(i + 1, see + 1))) {
      see++;
      if (see < w)
        dp[see]++;
    }
    // rep(j,w)cout << dp[j];cout << endl;
  }
  cout << h << endl;
}