#include <bits/stdc++.h>
using namespace std;
bool z[15][15];
int u[15][15];
int l[15][15];
int r, c, n, k;
bool f(int x, int x1, int y, int y1) {
  int s = 0;
  for (int i = y; i <= y1; i++) {
    s += u[x1][i] - u[x - 1][i];
  }
  return s >= k;
}
int main() {
  cin >> r >> c >> n >> k;
  int x, y;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    z[x][y] = 1;
  }
  x = 0;
  long long s = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      u[i][j] = u[i - 1][j];
      if (z[i][j]) u[i][j]++;
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = i; j <= r; j++) {
      for (int k = 1; k <= c; k++) {
        for (int k1 = k; k1 <= c; k1++) {
          if (f(i, j, k, k1)) s++;
        }
      }
    }
  }
  cout << s;
  return 0;
}
