#include <bits/stdc++.h>
using namespace std;
namespace io {
const int SI = 1 << 21 | 1;
char IB[SI], *IS, *IT, OB[SI], *OS = OB, *OT = OS + SI - 1, c, ch[100];
int f, t;
inline void flush() { fwrite(OB, 1, OS - OB, stdout), OS = OB; }
inline void pc(char x) {
  *OS++ = x;
  if (OS == OT) flush();
}
template <class I>
inline void rd(I &x) {
  for (f = 1, c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                              IS == IT ? EOF : *IS++)
                            : *IS++);
       c < '0' || c > '9';
       c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                       IS == IT ? EOF : *IS++)
                     : *IS++))
    if (c == '-') f = -1;
  for (x = 0; c >= '0' && c <= '9'; x = (x << 3) + (x << 1) + (c & 15),
      c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                      IS == IT ? EOF : *IS++)
                    : *IS++))
    ;
  x *= f;
}
template <class I>
inline void rda(I *a, int &n) {
  for (int i = 1; i <= n; i++) rd(a[i]);
}
inline void rds(char *s, int &x) {
  for (c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                       IS == IT ? EOF : *IS++)
                     : *IS++);
       c < 33 || c > 126;
       c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                       IS == IT ? EOF : *IS++)
                     : *IS++))
    ;
  for (x = 0; c >= 33 && c <= 126;
       s[++x] = c, c = (IS == IT ? (IT = (IS = IB) + fread(IB, 1, SI, stdin),
                                   IS == IT ? EOF : *IS++)
                                 : *IS++))
    ;
  s[x + 1] = '\0';
}
template <class I>
inline void print(I x, char k = '\n') {
  if (!x) pc('0');
  if (x < 0) pc('-'), x = -x;
  while (x) ch[++t] = x % 10 + '0', x /= 10;
  while (t) pc(ch[t--]);
  pc(k);
}
inline void prints(string s) {
  int x = s.length();
  while (t < x) pc(s[t++]);
  pc('\n'), t = 0;
}
struct Flush {
  ~Flush() { flush(); }
} flusher;
}  // namespace io
using io::print;
using io::prints;
using io::rd;
using io::rda;
using io::rds;
const int N = 3e3 + 7;
int r, c, n, k, p[N], q[N], t[N], v[N];
pair<int, int> a[N];
vector<pair<int, int> > R[N], C[N];
long long ans;
inline long long calc(int i, int j, int o) {
  long long ret = 1ll * (t[o] - j + 1) * (a[o].second - a[p[o]].second) *
                  (c - a[v[o]].second + 1);
  t[o] = j - 1;
  return ret;
}
int main() {
  rd(r), rd(c), rd(n), rd(k);
  for (int i = 1, x, y; i <= n; i++)
    rd(x), rd(y), a[i] = make_pair(x, y), R[x].push_back(make_pair(y, i)),
                  C[y].push_back(make_pair(x, i));
  for (int i = 1; i <= r; i++) sort(R[i].begin(), R[i].end());
  for (int i = 1; i <= c; i++) sort(C[i].begin(), C[i].end());
  a[n + 1] = make_pair(r + 1, c + 1);
  for (int i = 1; i <= r; i++) {
    int o = 0;
    for (int j = 1; j <= c; j++)
      for (pair<int, int> x : C[j])
        if (x.first >= i)
          q[o] = x.second, p[x.second] = o, o = x.second, t[o] = r, v[o] = 0;
    q[o] = q[n + 1] = v[n + 1] = n + 1, t[n + 1] = r, o = 0;
    for (int j = 1; j <= k; j++) o = q[o];
    for (int j = q[0]; j != n + 1; j = q[j], o = q[o]) v[j] = o;
    for (int j = r; j >= i; j--)
      for (pair<int, int> x : R[j]) {
        ans += calc(i, j, q[x.second]), o = x.second;
        for (int l = 1; o && l <= k; l++, o = p[o]) ans += calc(i, j, o);
        q[p[x.second]] = q[x.second], p[q[x.second]] = p[x.second];
        int w = o = q[o];
        for (int l = 1; l < k; l++) w = q[w];
        while (o != q[x.second]) v[o] = w, o = q[o], w = q[w];
      }
  }
  print(ans);
  return 0;
}
