#include <bits/stdc++.h>
const int INF = 0x7F7F7F7F;
const double EPS = 1e-10;
const long long mod7 = 1e9 + 7;
const long long mod9 = 1e9 + 9;
using namespace std;
inline long long rit() {
  long long f = 0, key = 1;
  char c;
  do {
    c = getchar();
    if (c == '-') key = -1;
  } while (c < '0' || c > '9');
  do {
    f = f * 10 + c - '0';
    c = getchar();
  } while (c >= '0' && c <= '9');
  return f * key;
}
void init() {}
int r, c, n, k;
int mk[100][100];
void read() {
  int i, x, y;
  r = rit(), c = rit(), n = rit(), k = rit();
  for (i = 0; i < n; i++) {
    x = rit(), y = rit();
    mk[x][y] = 1;
  }
}
int chk(int a, int b, int c, int d) {
  int i, j, cnt = 0;
  for (i = a; i <= c; i++) {
    for (j = b; j <= d; j++) {
      cnt += mk[i][j];
    }
  }
  return (cnt >= k);
}
void solve() {
  int i, j, ans = 0, i2, j2;
  for (i = 1; i <= r; i++) {
    for (j = 1; j <= c; j++) {
      for (i2 = i; i2 <= r; i2++) {
        for (j2 = j; j2 <= c; j2++) {
          if (chk(i, j, i2, j2)) ans++;
        }
      }
    }
  }
  cout << ans << endl;
}
int main() {
  int nn = 1;
  init();
  while (nn--) {
    read();
    solve();
  }
  return 0;
}
