#include <bits/stdc++.h>
using namespace std;
int r, c, n, k;
bool f[20][20];
int main() {
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 1; i <= n; ++i) {
    int x, y;
    scanf("%d%d", &x, &y);
    f[x][y] = 1;
  }
  int ans(0);
  for (int ix = 1; ix <= r; ++ix)
    for (int iy = 1; iy <= c; ++iy)
      for (int jx = ix; jx <= r; ++jx)
        for (int jy = iy; jy <= c; ++jy) {
          int cur(0);
          for (int i = ix; i <= jx; ++i)
            for (int j = iy; j <= jy; ++j) cur += f[i][j];
          ans += (cur >= k);
        }
  printf("%d\n", ans);
  return 0;
}
