#include <bits/stdc++.h>
using namespace std;
int r, c, n, m;
int ss[15][15];
bool J(int x1, int y1, int x2, int y2) {
  int s = 0;
  for (int i = x1; i <= x2; i++) {
    for (int j = y1; j <= y2; j++) {
      s += ss[i][j];
    }
  }
  return s >= m;
}
int main() {
  int x, y;
  while (~scanf("%d%d%d%d", &r, &c, &n, &m)) {
    memset(ss, 0, sizeof(ss));
    for (int i = 0; i < n; i++) {
      scanf("%d%d", &x, &y);
      ss[x][y] = 1;
    }
    int sum = 0;
    for (int i = 1; i <= r; i++) {
      for (int j = 1; j <= c; j++) {
        for (int k = 1; k <= r; k++) {
          for (int l = 1; l <= c; l++) {
            if (J(i, j, k, l)) sum++;
          }
        }
      }
    }
    printf("%d\n", sum);
  }
  return 0;
}
