#include <bits/stdc++.h>
using namespace std;
int a[15][15], b[15][15];
int main() {
  int n, m, k, h, i, j, i1, j1;
  scanf("%d%d%d%d", &n, &m, &k, &h);
  for (i = 1; i <= k; i++) {
    int l, r;
    scanf("%d%d", &l, &r);
    a[l][r] = 1;
  }
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= m; j++) {
      b[i][j] = b[i - 1][j] + b[i][j - 1] - b[i - 1][j - 1] + a[i][j];
    }
  }
  int ans = 0;
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= m; j++) {
      for (i1 = i; i1 <= n; i1++) {
        for (j1 = j; j1 <= m; j1++) {
          if (b[i1][j1] - b[i - 1][j1] - b[i1][j - 1] + b[i - 1][j - 1] >= h)
            ans++;
        }
      }
    }
  }
  printf("%d", ans);
  return 0;
}
