#include <bits/stdc++.h>
using namespace std;
bool v[15][15];
int main() {
  ios_base::sync_with_stdio(false);
  int n, m, k, i, j, x, r, c, i2, j2;
  long long s = 0;
  cin >> n >> m >> x >> k;
  for (i = 1; i <= x; i++) {
    cin >> r >> c;
    v[r][c] = 1;
  }
  for (i = 1; i <= n; i++)
    for (j = 1; j <= m; j++) {
      for (r = 1; i + r - 1 <= n; r++)
        for (c = 1; j + c - 1 <= m; c++) {
          int s2 = 0;
          for (i2 = i; i2 <= i + r - 1; i2++)
            for (j2 = j; j2 <= j + c - 1; j2++) s2 += v[i2][j2];
          if (s2 >= k) s++;
        }
    }
  cout << s;
  return 0;
}
