#include <bits/stdc++.h>
using namespace std;
int ans = 0;
int k;
const int N = 11;
int member[N][N];
int sum[N][N][N][N];
void build(int rowStart, int rowEnd, int colStart, int colEnd) {
  if (sum[rowStart][rowEnd][colStart][colEnd] != -1) {
    return;
  }
  if (rowStart != rowEnd) {
    build(rowStart, rowEnd - 1, colStart, colEnd);
    build(rowEnd, rowEnd, colStart, colEnd);
    sum[rowStart][rowEnd][colStart][colEnd] =
        sum[rowStart][rowEnd - 1][colStart][colEnd] +
        sum[rowEnd][rowEnd][colStart][colEnd];
  } else if (colStart != colEnd) {
    build(rowStart, rowEnd, colStart, colEnd - 1);
    build(rowStart, rowEnd, colEnd, colEnd);
    sum[rowStart][rowEnd][colStart][colEnd] =
        sum[rowStart][rowEnd][colStart][colEnd - 1] +
        sum[rowStart][rowEnd][colEnd][colEnd];
  }
  if (sum[rowStart][rowEnd][colStart][colEnd] >= k) {
    ++ans;
  }
}
int main() {
  int r, c, n;
  cin >> r >> c >> n >> k;
  for (int i = 1; i <= r; ++i) {
    for (int j = 1; j <= c; ++j) {
      member[i][j] = 0;
    }
  }
  int x, y;
  for (int i = 0; i < n; ++i) {
    cin >> x >> y;
    member[x][y] = 1;
  }
  for (int i = 1; i <= r; ++i) {
    for (int j = 1; j <= r; ++j) {
      for (int kk = 1; kk <= c; ++kk) {
        for (int l = 1; l <= c; ++l) {
          if (i == j && kk == l) {
            sum[i][j][kk][l] = member[i][kk];
            if (sum[i][j][kk][l] >= k) {
              ++ans;
            }
          } else {
            sum[i][j][kk][l] = -1;
          }
        }
      }
    }
  }
  for (int i = 1; i <= r; ++i) {
    for (int j = i; j <= r; ++j) {
      for (int kk = 1; kk <= c; ++kk) {
        for (int l = kk; l <= c; ++l) {
          build(i, j, kk, l);
        }
      }
    }
  }
  cout << ans;
}
