#include <bits/stdc++.h>
using namespace std;
int main() {
  int r, c, n, k, x, y;
  scanf("%d", &r);
  scanf("%d", &c);
  scanf("%d", &n);
  scanf("%d", &k);
  int a[r][c];
  for (int i = 0; i < r; i++)
    for (int j = 0; j < c; j++) a[i][j] = 0;
  for (int i = 0; i < n; i++) {
    scanf("%d", &x);
    scanf("%d", &y);
    x--;
    y--;
    a[x][y] = 1;
  }
  int count = 0, ans = 0;
  for (int i = 0; i < r; i++) {
    for (int j = 0; j < c; j++) {
      for (int kk = i; kk < r; kk++) {
        for (int l = j; l < c; l++) {
          count = 0;
          for (int m = i; m <= kk; m++) {
            for (int nn = j; nn <= l; nn++) {
              if (a[m][nn]) count++;
            }
          }
          if (count >= k) ans++;
        }
      }
    }
  }
  printf("%d\n", ans);
  return 0;
}
