#include <bits/stdc++.h>
using namespace std;
struct lt {
  int x, y;
} zb[12];
int ans = 0;
bool pd(int y1, int x1, int y2, int x2, int k, int n) {
  int sum = 0;
  for (int i = 0; i < n; ++i) {
    if (zb[i].x >= x1 && zb[i].x <= x2 && zb[i].y >= y1 && zb[i].y <= y2) ++sum;
    if (sum == k) return true;
  }
  return false;
}
void wosou(int y, int x, int r, int c, int k, int n) {
  for (int i = y; i <= r; ++i)
    for (int j = x; j <= c; ++j) {
      if (pd(y, x, i, j, k, n)) ans++;
    }
}
int main() {
  int r, c, n, k;
  while (~scanf("%d%d%d%d", &r, &c, &n, &k)) {
    ans = 0;
    for (int i = 0; i < n; ++i) scanf("%d%d", &zb[i].y, &zb[i].x);
    for (int i = 1; i <= r; ++i)
      for (int j = 1; j <= c; ++j) {
        wosou(i, j, r, c, k, n);
      }
    printf("%d\n", ans);
  }
  return 0;
}
