#include <bits/stdc++.h>
using namespace std;
bool used[11][11];
bool is(int a, int b, int c, int d, int m) {
  int cnt = 0;
  for (int i = a; i <= c; i++) {
    for (int j = b; j <= d; j++) {
      cnt += used[i][j];
    }
  }
  return cnt >= m;
}
int main() {
  int n, m, a = 0, z, c, x, y;
  cin >> z >> c >> n >> m;
  for (int i = 1; i <= n; i++) {
    cin >> x >> y;
    used[x][y] = true;
  }
  for (int i = 1; i <= z; i++) {
    for (int j = 1; j <= c; j++) {
      for (int l = i; l <= z; l++) {
        for (int k = j; k <= c; k++) {
          if (is(i, j, l, k, m) == 1) {
            a++;
          }
        }
      }
    }
  }
  cout << a;
}
