#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using VS = vector<string>;
using VB = vector<bool>;
using VVI = vector<VI>;
using VVL = vector<VL>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;
using TI3 = std::tuple<int, int, int>;
using TI4 = std::tuple<int, int, int, int>;
using TL3 = std::tuple<ll, ll, ll>;
using TL4 = std::tuple<ll, ll, ll, ll>;
const int mod1 = 1e9 + 7, mod2 = 998244353, mod3 = 1e9 + 9;
const int mod = mod1;
const ll inf = 1e18;
const string wsp = " ";
const string tb = "\t";
const string rt = "\n";
template <typename T>
void show1dvec(const vector<T> &v) {
  if (v.size() == 0) return;
  int n = v.size() - 1;
  for (int i = 0; i < (int)(n); i++) cout << v[i] << wsp;
  cout << v[n] << rt;
  return;
}
template <typename T>
void show2dvec(const vector<vector<T>> &v) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++) show1dvec(v[i]);
}
template <typename T, typename S>
void show1dpair(const vector<pair<T, S>> &v) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++)
    cout << v[i].first << wsp << v[i].second << rt;
  return;
}
template <typename T, typename S>
void pairzip(const vector<pair<T, S>> &v, vector<T> &t, vector<T> &s) {
  int n = v.size();
  for (int i = 0; i < (int)(n); i++) {
    t.push_back(v[i].first);
    s.push_back(v[i].second);
  }
  return;
}
template <typename T>
void maxvec(vector<T> &v) {
  T s = v[0];
  int n = v.size();
  for (int i = 0; i < (int)(n - 1); i++) {
    if (s > v[i + 1]) {
      v[i + 1] = s;
    }
    s = v[i + 1];
  }
}
template <typename T, typename S>
bool myfind(T t, S s) {
  return find(t.cbegin(), t.cend(), s) != t.cend();
}
bool check(int y, int x, int h, int w) {
  return 0 <= y && y < h && 0 <= x && x < w;
}
template <typename T>
vector<T> cumsum(const vector<T> &v) {
  T s = 0;
  vector<T> ret;
  for (int i = 0; i < (int)(v.size()); i++) {
    s += v[i];
    s %= mod;
    ret.emplace_back(s);
  }
  return ret;
}
bool iskadomatsu(int a, int b, int c) {
  return (a != b && b != c && c != a) && ((a > b && b < c) || (a < b && b > c));
}
VS split(string s, char c) {
  VS ret;
  string part;
  s += c;
  for (int i = 0; i < (int)(s.length()); i++) {
    if (s[i] == c) {
      ret.emplace_back(part);
      part = "";
    } else if (s[i] != c) {
      part += s[i];
    }
  }
  return ret;
}
string removezero(string &s) {
  string ret;
  for (auto z : s)
    if (z != '0') ret += z;
  return ret;
}
ll sumdigit(ll x) {
  ll ans{0};
  while (x > 0) {
    ans += x % 10;
    x /= 10;
  }
  return ans;
}
template <typename T, typename S, typename R>
ll pow_mod(T p, S q, R mod = 1ll) {
  ll ret = 1, r = p;
  while (q) {
    if (q % 2) ret *= r, ret %= mod;
    r = (r * r) % mod, q /= 2;
  }
  return ret % mod;
}
template <typename T, typename S>
ll pow_no_mod(T p, S q) {
  ll ret = 1, r = p;
  while (q) {
    if (q % 2) ret *= r;
    r = (r * r), q /= 2;
  }
  return ret;
}
void make_frac_tables(VL &frac_list, VL &frac_inv_list) {
  for (int i = 0; i < (int)(frac_list.size() - 1); i++) {
    frac_list[i + 1] *= frac_list[i] * (i + 1);
    frac_list[i + 1] %= mod;
    frac_inv_list[i + 1] *= frac_inv_list[i] * pow_mod(i + 1, mod - 2, mod);
    frac_inv_list[i + 1] %= mod;
  }
}
ll comb(int a, int b, const VL &frac_list, const VL &frac_inv_list) {
  if (a < b) return 0;
  if (b < 0) return 0;
  ll ret = frac_list[a];
  ret *= frac_inv_list[b];
  ret %= mod;
  ret *= frac_inv_list[a - b];
  ret %= mod;
  return ret;
}
int h, w, k, k2, q, x, y, t;
vector<TI3> events;
bool isok(int ti) {
  VVI field(h - k + 3, VI(w - k + 3, 0));
  int uy, ux, dy, dx, fieldmax{0};
  for (int _i = 0; _i < (int)(q); _i++) {
    auto [y, x, t] = events[_i];
    if (t <= ti) {
      uy = max(0, y - k + 1);
      ux = max(0, x - k + 1);
      dy = min(h - k + 2, y + 1);
      dx = min(w - k + 2, x + 1);
      field[uy][ux]++;
      field[uy][dx]--;
      field[dy][ux]--;
      field[dy][dx]++;
    }
  }
  for (int i = 0; i < (int)(h - k + 3); i++) {
    for (int j = 0; j < (int)(w - k + 2); j++) {
      field[i][j + 1] += field[i][j];
    }
  }
  for (int j = 0; j < (int)(w - k + 3); j++) {
    for (int i = 0; i < (int)(h - k + 2); i++) {
      field[i + 1][j] += field[i][j];
    }
  }
  for (int i = 0; i < (int)(h - k + 2); i++)
    for (int j = 0; j < (int)(w - k + 2); j++)
      fieldmax = max(fieldmax, field[i][j]);
  return fieldmax == k2;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int ok, mid, ng, tmax{1000000000};
  cin >> h >> w >> k >> q;
  events.resize(q);
  k2 = k * k;
  for (int i = 0; i < (int)(q); i++) {
    cin >> y >> x >> t;
    events[i] = make_tuple(y - 1, x - 1, t);
  }
  if (!isok(tmax)) {
    cout << -1 << rt;
    return 0;
  }
  ok = tmax;
  ng = -1;
  while (ok - ng > 1) {
    mid = (ok + ng) / 2;
    if (isok(mid)) {
      ok = mid;
    } else {
      ng = mid;
    }
  }
  cout << ok << rt;
  return 0;
}
