#include <bits/stdc++.h>
using namespace std;
int r, i, x, j, d, z, l, o, sum, nrsol, y, c, n, k, a[11][11];
int main() {
  cin >> r >> c >> n >> k;
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (i = 1; i <= r; i++)
    for (j = 1; j <= c; j++) {
      for (d = i; d <= r; d++)
        for (z = j; z <= c; z++) {
          sum = 0;
          for (l = i; l <= d; l++)
            for (o = j; o <= z; o++) sum += a[l][o];
          if (sum >= k) nrsol++;
        }
    }
  cout << nrsol;
  return 0;
}
