#include <bits/stdc++.h>
using namespace std;
const int maxn = 15;
inline void read(int &x) {
  x = 0;
  int f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  x *= f;
}
inline void judge() {
  freopen("in.txt", "r", stdin);
  freopen("out.txt", "w", stdout);
}
int sum[maxn][maxn];
int a[maxn][maxn], n, m, l, k;
int main() {
  read(n);
  read(m);
  read(l);
  read(k);
  for (int i = (1); i <= (l); i++) {
    int x, y;
    read(x);
    read(y);
    a[x][y] = 1;
  }
  int ans = 0;
  for (int i = (1); i <= (n); i++)
    for (int j = (1); j <= (m); j++)
      sum[i][j] = (sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1]) + a[i][j];
  for (int x1 = (1); x1 <= (n); x1++)
    for (int y1 = (1); y1 <= (m); y1++)
      for (int x2 = (x1); x2 <= (n); x2++)
        for (int y2 = (y1); y2 <= (m); y2++) {
          int tot = (sum[x2][y2] - sum[x1 - 1][y2] - sum[x2][y1 - 1] +
                     sum[x1 - 1][y1 - 1]);
          if (tot >= k) ans++;
        }
  cout << ans;
  return 0;
}
