#include <bits/stdc++.h>
using namespace std;
int toNumber(string s) {
  int Number;
  if (!(istringstream(s) >> Number)) Number = 0;
  return Number;
}
string toString(int number) {
  ostringstream ostr;
  ostr << number;
  return ostr.str();
}
int main() {
  long long r, c, n, k;
  cin >> r >> c >> n >> k;
  int t[r][c];
  for (int i = 0; i < (int)(r); i++) {
    for (int j = 0; j < (int)(c); j++) {
      t[i][j] = 0;
    }
  }
  for (int i = 0; i < (int)(n); i++) {
    int x, y;
    cin >> x >> y;
    t[x - 1][y - 1] = 1;
  }
  long long pos = 0;
  for (int i1 = 0; i1 < (int)(r); i1++) {
    for (int i2 = (i1); i2 < (int)(r + 1); i2++) {
      for (int j1 = 0; j1 < (int)(c); j1++) {
        for (int j2 = (j1); j2 < (int)(c + 1); j2++) {
          int cant = 0;
          for (int aa = (i1); aa < (int)(i2); aa++) {
            for (int bb = (j1); bb < (int)(j2); bb++) {
              cant += t[aa][bb];
            }
          }
          if (cant >= k) {
            pos++;
          }
        }
      }
    }
  }
  cout << pos;
}
