#include <bits/stdc++.h>
using namespace std;
const int MAXN = 10;
int p[MAXN + 5][MAXN + 5];
int n, kk, first, second, sol;
bool f(int a, int b, int c, int d) {
  int sum = 0;
  for (int i = (a), _for = (c + 1); i < _for; ++i)
    for (int j = (b), _for = (d + 1); j < _for; ++j) sum += p[i][j];
  return sum >= kk;
}
int main() {
  scanf("%d%d%d%d", &first, &second, &n, &kk);
  for (int i = (0), _for = (n); i < _for; ++i) {
    int a, b;
    scanf("%d%d", &a, &b);
    a--;
    b--;
    p[a][b] = 1;
  }
  for (int i = (0), _for = (first); i < _for; ++i)
    for (int j = (0), _for = (second); j < _for; ++j)
      for (int k = (i), _for = (first); k < _for; ++k)
        for (int l = (j), _for = (second); l < _for; ++l)
          if (f(i, j, k, l)) sol++;
  printf("%d\n", sol);
  return 0;
}
