#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e9;
const long long N = 1e1 + 1;
const long long mod = 1e9 + 7;
const long double eps = 1E-7;
int ans;
int x, y;
int d[N][N];
int n, m, f, k;
int sum(int a, int b, int x, int y) {
  return d[x][y] + d[a - 1][b - 1] - d[a - 1][y] - d[x][b - 1];
}
int main() {
  ios_base::sync_with_stdio(0);
  cin >> n >> m;
  cin >> f >> k;
  for (int i = 1; i <= f; i++) {
    cin >> x >> y;
    d[x][y]++;
  }
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      d[i][j] += d[i - 1][j] + d[i][j - 1] - d[i - 1][j - 1];
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++)
      for (int h = i; h <= n; h++)
        for (int g = j; g <= m; g++)
          if (sum(i, j, h, g) >= k) ans = 1 + ans;
  cout << ans << endl;
}
