#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1.0);
const double eps = 1e-11;
template <class T>
T sqr(const T &x) {
  return x * x;
}
template <class T>
T lowbit(const T &x) {
  return (x ^ (x - 1)) & x;
}
inline long long mlog2(long long n) {
  long long c = 0;
  c += 64 - __builtin_clzll(n);
  return c - 1;
}
void fun(int a, int b) {
  for (b = 0; b < 5; b++) a++;
}
bool arr[15][15] = {0};
int k;
bool check(int a, int b, int c, int d) {
  int i, j, tp = 0;
  for (i = a; i <= b; i++)
    for (j = c; j <= d; j++) {
      if (arr[i][j] == 1) tp++;
      if (tp >= k) return 1;
    }
  return 0;
}
int main() {
  int r, c, n, i, j, p, q, cou = 0, x, y;
  cin >> r >> c >> n >> k;
  for (i = 0; i < n; i++) {
    cin >> x >> y;
    x--;
    y--;
    arr[x][y] = 1;
  }
  for (i = 0; i < r; i++) {
    for (j = i; j < r; j++) {
      for (p = 0; p < c; p++) {
        for (q = p; q < c; q++) {
          if (check(i, j, p, q)) cou++;
        }
      }
    }
  }
  cout << cou;
}
