#include <bits/stdc++.h>
using namespace std;
int v[15][15], r, c, n, k;
int sol;
int dreptunghi(int i1, int j1, int i2, int j2) {
  int i, j, a = 0;
  for (i = i1; i <= i2; i++)
    for (j = j1; j <= j2; j++) a += v[i][j];
  return a;
}
int main() {
  int i, j;
  int x, y;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (i = 1; i <= n; i++) {
    scanf("%d %d", &x, &y);
    v[x][y] = 1;
  }
  int i1, j1, i2, j2;
  for (i1 = 1; i1 <= r; i1++)
    for (j1 = 1; j1 <= c; j1++)
      for (i2 = i1; i2 <= r; i2++)
        for (j2 = j1; j2 <= c; j2++)
          if (dreptunghi(i1, j1, i2, j2) >= k) sol++;
  printf("%d", sol);
  return 0;
}
