#include <bits/stdc++.h>
using namespace std;
int n, m, k, c;
int v[22][22], ans;
void get(int a, int b, int c, int d) {
  int j = 0;
  for (int i = a; i <= c; i++)
    for (int h = b; h <= d; h++) j += v[i][h];
  if (j >= k) ans++;
}
void tap(int a, int b) {
  for (int i = a; i <= n; i++)
    for (int h = b; h <= m; h++) get(a, b, i, h);
}
int main() {
  cin >> n >> m >> c >> k;
  while (c--) {
    int a, b;
    cin >> a >> b;
    v[a][b] = 1;
  }
  for (int i = 1; i <= n; i++)
    for (int h = 1; h <= m; h++) tap(i, h);
  cout << ans;
}
