#include <bits/stdc++.h>
using namespace std;
char mp[16][16];
int sum[16][16];
int main() {
  int r, c, n, ca;
  while (scanf("%d%d%d%d", &r, &c, &n, &ca) != EOF) {
    for (int i = 1; i <= r; i++)
      for (int j = 1; j <= c; j++) mp[i][j] = '*';
    for (int i = 0; i < n; i++) {
      int x, y;
      scanf("%d%d", &x, &y);
      mp[x][y] = '#';
    }
    int ans = 0;
    for (int i = 1; i <= r; i++) {
      for (int j = 1; j <= c; j++) {
        for (int k = 1; k <= i; k++) {
          for (int l = 1; l <= j; l++) {
            int cnt = 0;
            for (int o = k; o <= i; o++) {
              for (int p = l; p <= j; p++) {
                if (mp[o][p] == '#') cnt++;
              }
            }
            if (cnt >= ca) ans++;
          }
        }
      }
    }
    printf("%d\n", ans);
  }
}
