#include <bits/stdc++.h>
using namespace std;
int A[15][15];
int main() {
  int r, c, n, q, counter = 0;
  scanf("%d%d%d%d", &r, &c, &n, &q);
  int x, y;
  for (int i = 0; i < n; i++) {
    scanf("%d%d", &x, &y);
    A[x - 1][y - 1] = 1;
  }
  for (int i = 0; i < r; i++)
    for (int j = 0; j < c; j++) {
      if (i > 0) A[i][j] += A[i - 1][j];
      if (j > 0) A[i][j] += A[i][j - 1];
      if (i > 0 && j > 0) A[i][j] -= A[i - 1][j - 1];
    }
  long long subRect;
  for (int i = 0; i < r; i++)
    for (int j = 0; j < c; j++)
      for (int k = i; k < r; k++)
        for (int l = j; l < c; l++) {
          subRect = A[k][l];
          if (i > 0) subRect -= A[i - 1][l];
          if (j > 0) subRect -= A[k][j - 1];
          if (i > 0 && j > 0) subRect += A[i - 1][j - 1];
          if (subRect >= q) counter++;
        }
  printf("%d", counter);
  return 0;
}
