#include <bits/stdc++.h>
using namespace std;
int ans, fin, r, p, c, n, k, i, j, l, u, v, x, y;
int main() {
  cin >> r >> c >> n >> p;
  char str[50][50];
  for (i = 1; i <= n; i++) {
    cin >> x >> y;
    str[x][y] = '#';
  }
  for (i = 1; i <= r; i++)
    for (j = i; j <= r; j++)
      for (k = 1; k <= c; k++)
        for (l = k; l <= c; l++) {
          ans = 0;
          for (u = i; u <= j; u++)
            for (v = k; v <= l; v++)
              if (str[u][v] == '#') ++ans;
          if (ans >= p) ++fin;
        }
  cout << fin;
  return 0;
}
