#include <bits/stdc++.h>
using namespace std;
long long r, c, n, k, ans, a[100][100];
int main() {
  cin >> r >> c >> n >> k;
  for (long long i = 1; i <= n; i++) {
    long long x, y;
    cin >> x >> y;
    a[x][y]++;
  }
  for (long long i = 1; i <= r; i++) {
    for (long long j = 1; j <= c; j++) {
      a[i][j] += a[i - 1][j] + a[i][j - 1] - a[i - 1][j - 1];
    }
  }
  for (long long x1 = 1; x1 <= r; x1++) {
    for (long long Y1 = 1; Y1 <= c; Y1++) {
      for (long long x2 = x1; x2 <= r; x2++) {
        for (long long y2 = Y1; y2 <= c; y2++) {
          if (a[x2][y2] - a[x1 - 1][y2] - a[x2][Y1 - 1] + a[x1 - 1][Y1 - 1] >=
              k)
            ans++;
        }
      }
    }
  }
  cout << ans;
  return 0;
}
