public static void Main()
{
var nmkq = Console.ReadLine().Split().Select(int.Parse).ToArray();
var n = nmkq[0];
var m = nmkq[1];
var k = nmkq[2];
var q = nmkq[3];
int[][] mat = Enumerable.Repeat(0, n).Select(_ => Enumerable.Repeat(int.MaxValue / 2, m).ToArray()).ToArray();
for (int i = 0; i < q; i++)
{
var xyt = Console.ReadLine().Split().Select(int.Parse).ToArray();
var x = xyt[0] - 1;
var y = xyt[1] - 1;
var t = xyt[2];
mat[x][y] = t;
}
FoldableQueue <int>[] fqueues = Enumerable.Repeat(0, n).Select(_ => new FoldableQueue <int>(Max)).ToArray();
int res = int.MaxValue / 2;
for (int j = 0; j < m; j++)
{
FoldableQueue <int> fqueue = new FoldableQueue <int>(Max);
for (int i = 0; i < n; i++)
{
fqueues[i].Push(mat[i][j]);
if (fqueues[i].Count == k)
{
fqueue.Push(fqueues[i].Value);
fqueues[i].Pop();
}
if (fqueue.Count == k)
{
res = Min(res, fqueue.Value);
fqueue.Pop();
}
}
}
if (res == int.MaxValue / 2)
{
res = -1;
}
Console.WriteLine(res);
}
static void Solve()
{
int n = int.Parse(Console.ReadLine());
var a = Console.ReadLine().Split().Select(int.Parse).ToArray();
bool Validate(int[] a)
{
return(a.Min() == 1 && a.Max() == a.Length && a.Distinct().Count() == a.Length);
}
int valid = n + 1;
int invalid = 1;
while (valid - invalid > 1)
{
var mid = (invalid + valid) / 2;
var cnt = n - mid + 1;
var seq = new int[cnt];
FoldableQueue <int> fQueue = new FoldableQueue <int>(Min);
for (int i = 0; i < mid - 1; i++)
{
fQueue.Push(a[i]);
}
for (int i = 0; i < seq.Length; i++)
{
fQueue.Push(a[i + mid - 1]);
seq[i] = fQueue.Value;
fQueue.Pop();
}
if (Validate(seq))
{
valid = mid;
}
else
{
invalid = mid;
}
}
char[] res = new char[n];
res[0] = Validate(a) ? '1' : '0';
for (int i = 1; i < n; i++)
{
res[i] = valid <= i + 1 ? '1' : '0';
}
Console.WriteLine(res);
}