#include <bits/stdc++.h>
using namespace std;
bool debug = 0;
int n, m, k;
int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
string direc = "RDLU";
long long ln, lk, lm;
int r, c;
vector<int> v[3105];
multiset<int> ms;
int pos[30];
void fmain() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 0, x, y; i < n; i++) {
    scanf("%d%d", &x, &y);
    v[x].push_back(y);
  }
  long long ans = 0, cnt = 0;
  for (int(i) = 1; (i) <= (int)(r); (i)++) {
    ms.clear();
    cnt = 0;
    for (int(j) = 0; (j) < (int)(k + 1); (j)++) {
      ms.insert(0);
      ms.insert(c + 1);
    }
    for (int j = i; j <= r; j++) {
      for (int x : v[j]) {
        auto it = ms.insert(x), it1 = it;
        for (int z = k; z >= 0; z--) {
          pos[z] = *it;
          it--;
        }
        it1++;
        for (int z = k + 1; z <= k + k; z++) {
          pos[z] = *it1;
          it1++;
        }
        for (int z = k; z < k + k; z++) {
          cnt += (long long)(pos[z + 1] - pos[z]) *
                 (pos[z - (k - 1)] - pos[z - k]);
        }
      }
      ans += cnt;
    }
  }
  printf("%lld\n", ans);
}
int main() {
  fmain();
  return 0;
}
