#include <bits/stdc++.h>
using namespace std;
int t[20][20], s[20][20];
int main() {
  int r, c, k, n, x, y, ans = 0;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int j = 1; j <= n; j++) {
    scanf("%d%d", &x, &y);
    t[x][y] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      s[i][j] = s[i - 1][j] + s[i][j - 1] - s[i - 1][j - 1] + t[i][j];
  for (int x1 = 1; x1 <= r; x1++)
    for (int y1 = 1; y1 <= c; y1++)
      for (int x2 = x1; x2 <= r; x2++)
        for (int y2 = y1; y2 <= c; y2++)
          if (s[x2][y2] - s[x1 - 1][y2] - s[x2][y1 - 1] + s[x1 - 1][y1 - 1] >=
              k)
            ans++;
  printf("%d", ans);
}
