#include <bits/stdc++.h>
using namespace std;
template <typename T1, typename T2>
void chkmin(T1 &x, T2 y) {
  if (x > y) x = y;
}
template <typename T1, typename T2>
void chkmax(T1 &x, T2 y) {
  if (x < y) x = y;
}
namespace fastio {
char rbuf[1 << 23], *p1 = rbuf, *p2 = rbuf, wbuf[1 << 23], *p3 = wbuf;
inline char getc() {
  return p1 == p2 &&
                 (p2 = (p1 = rbuf) + fread(rbuf, 1, 1 << 23, stdin), p1 == p2)
             ? -1
             : *p1++;
}
inline void putc(char x) { (*p3++ = x); }
template <typename T>
void read(T &x) {
  x = 0;
  char c = getchar();
  T neg = 0;
  while (!isdigit(c)) neg |= !(c ^ '-'), c = getchar();
  while (isdigit(c)) x = (x << 3) + (x << 1) + (c ^ 48), c = getchar();
  if (neg) x = (~x) + 1;
}
template <typename T>
void recursive_print(T x) {
  if (!x) return;
  recursive_print(x / 10);
  putc(x % 10 ^ 48);
}
template <typename T>
void print(T x) {
  if (!x) putc('0');
  if (x < 0) putc('-'), x = ~x + 1;
  recursive_print(x);
}
void print_final() { fwrite(wbuf, 1, p3 - wbuf, stdout); }
}  // namespace fastio
const int MAXN = 3e3;
int R, C, n, k, x[MAXN + 5], y[MAXN + 5];
vector<int> vr[MAXN + 5], vc[MAXN + 5];
int l[MAXN + 5], r[MAXN + 5];
long long ans = 0;
int main() {
  scanf("%d%d%d%d", &R, &C, &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d%d", &x[i], &y[i]);
    vr[x[i]].push_back(i);
    vc[y[i]].push_back(i);
  }
  for (int i = 1; i <= MAXN; i++)
    sort(vr[i].begin(), vr[i].end(),
         [](int lhs, int rhs) { return y[lhs] > y[rhs]; });
  for (int i = 1; i <= MAXN; i++)
    sort(vc[i].begin(), vc[i].end(),
         [](int lhs, int rhs) { return x[lhs] < x[rhs]; });
  for (int i = 1; i <= R; i++) {
    memset(l, 0, sizeof(l));
    memset(r, 0, sizeof(r));
    int pre = 0;
    for (int j = 1; j <= C; j++)
      for (int t = 0; t < vc[j].size(); t++)
        if (x[vc[j][t]] >= i) {
          int id = vc[j][t];
          l[id] = pre;
          if (pre) r[pre] = id;
          pre = id;
        }
    long long ret = 0;
    for (int j = 1; j <= C; j++)
      for (int t = 0; t < vc[j].size(); t++)
        if (x[vc[j][t]] >= i) {
          int id = vc[j][t], cur = id;
          for (int stp = 1; stp < k; stp++) cur = r[cur];
          if (cur) ret += 1ll * (C - y[cur] + 1) * (y[id] - y[l[id]]);
        }
    ans += ret;
    for (int j = R; j >= i; j--) {
      for (int t = 0; t < vr[j].size(); t++) {
        int id = vr[j][t];
        if (!r[id]) {
          int cur = id;
          for (int stp = 1; stp < k; stp++) cur = l[cur];
          if (cur) ret -= 1ll * (y[cur] - y[l[cur]]) * (C - y[id] + 1);
          if (r[id]) l[r[id]] = l[id];
          if (l[id]) r[l[id]] = r[id];
          continue;
        }
        int cr = r[id], cl = r[id], stp = 0;
        while (stp < k - 1) {
          if (!r[cr]) break;
          cr = r[cr];
          ++stp;
        }
        for (int o = 1; o <= k - 1 - stp; o++) cl = l[cl];
        if (cl) ret -= 1ll * (C - y[cr] + 1) * (y[cl] - y[l[cl]]);
        for (int o = 1; o <= stp + 1; o++) {
          cr = l[cr];
          cl = l[cl];
          if (cl) ret -= 1ll * (C - y[cr] + 1) * (y[cl] - y[l[cl]]);
        }
        if (r[id]) l[r[id]] = l[id];
        if (l[id]) r[l[id]] = r[id];
        if (!r[id]) continue;
        cr = r[id], cl = r[id], stp = 0;
        while (stp < k - 1) {
          if (!r[cr]) break;
          cr = r[cr];
          ++stp;
        }
        for (int o = 1; o <= k - 1 - stp; o++) cl = l[cl];
        if (cl) ret += 1ll * (C - y[cr] + 1) * (y[cl] - y[l[cl]]);
        for (int o = 1; o <= stp; o++) {
          cr = l[cr];
          cl = l[cl];
          if (cl) ret += 1ll * (C - y[cr] + 1) * (y[cl] - y[l[cl]]);
        }
      }
      ans += ret;
    }
  }
  printf("%lld\n", ans);
  return 0;
}
