#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
int a[10 + 30][10 + 30];
int dp[10 + 30][10 + 30];
int main() {
  cin >> r >> c >> n >> k;
  int ans = 0;
  int z, x;
  for (int i = 0; i < n; i++) {
    cin >> x >> z;
    a[x][z] = 1;
  }
  dp[1][1] = a[1][1];
  for (int i = 2; i <= r; i++) {
    dp[i][1] = dp[i - 1][1] + a[i][1];
  }
  for (int j = 2; j <= c; j++) {
    dp[1][j] = dp[1][j - 1] + a[1][j];
  }
  for (int i = 2; i <= r; i++) {
    for (int j = 2; j <= c; j++) {
      dp[i][j] = dp[i - 1][j] + dp[i][j - 1] - dp[i - 1][j - 1] + a[i][j];
    }
  }
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int p = i; p <= r; p++) {
        for (int o = j; o <= c; o++) {
          if (dp[p][o] - dp[i - 1][o] - dp[p][j - 1] + dp[i - 1][j - 1] >= k) {
            ans++;
          }
        }
      }
    }
  }
  cout << ans;
}
