#include <bits/stdc++.h>
using namespace std;
const int MAXN = 12;
int rows, cols, k, n;
char mat[MAXN][MAXN];
int sol = 0;
int Calc(int a, int b, int c, int d) {
  int ret = 0;
  for (int i = a; i <= c; i++) {
    for (int j = b; j <= d; j++) ret += (mat[i][j] == '#');
  }
  return ret;
}
int main() {
  scanf("%d %d %d %d", &rows, &cols, &n, &k);
  for (int i = 0; i <= rows; i++)
    for (int j = 0; j <= cols; j++) mat[i][j] = '.';
  for (int i = 0; i < n; i++) {
    int x, y;
    scanf("%d %d", &x, &y);
    mat[x][y] = '#';
  }
  for (int i = 1; i <= rows; i++) {
    for (int j = 1; j <= cols; j++) {
      for (int ii = i; ii <= rows; ii++) {
        for (int jj = j; jj <= cols; jj++) {
          int sum = Calc(i, j, ii, jj);
          sol += (sum >= k);
        }
      }
    }
  }
  printf("%d\n", sol);
  return 0;
}
