#include <bits/stdc++.h>
using namespace std;
inline int read() {
  char ch = getchar();
  int res = 0, flag = 1;
  while (ch < '0' || ch > '9') {
    if (ch == '-') flag = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    res = res * 10 + ch - '0';
    ch = getchar();
  }
  return res * flag;
}
int r, c, n, k, p[3005][3005], x, y, cnt[3005], nxt[3005], lst[3005], tmp[3005];
long long ans;
int main() {
  r = read();
  c = read();
  n = read();
  k = read();
  for (register int i = 1; i <= n; ++i) {
    x = read();
    y = read();
    p[x][++p[x][0]] = y;
  }
  for (register int i = 1; i <= r; ++i) sort(p[i] + 1, p[i] + p[i][0] + 1);
  for (register int i = r; i; --i) {
    int h = 1, t = 0, s = 0;
    long long now = 0;
    for (register int j = 1; j <= p[i][0]; ++j) ++cnt[p[i][j]];
    while (1) {
      while (t < c && s < k) {
        ++t;
        s += cnt[t];
      }
      if (s < k) break;
      while (h <= t && s >= k) {
        s -= cnt[h];
        ++h;
        now += c - t + 1;
      }
    }
    nxt[0] = 1;
    lst[c + 1] = c;
    for (register int j = 1; j <= c; ++j) {
      nxt[j] = j + 1;
      lst[j] = j - 1;
    }
    for (register int j = 1; j <= c; ++j)
      if (!cnt[j]) {
        nxt[lst[j]] = nxt[j];
        lst[nxt[j]] = lst[j];
      }
    memcpy(tmp, cnt, sizeof(cnt));
    for (register int j = r; j >= i; --j) {
      ans += now;
      for (register int u = 1; u <= p[j][0]; ++u) {
        h = t = p[j][u];
        int s1 = tmp[h], s2 = 0;
        while (nxt[t] <= c && s1 + s2 + tmp[nxt[t]] <= k) {
          t = nxt[t];
          s2 += tmp[t];
        }
        while (1) {
          if (s1 + s2 == k) now -= (h - lst[h]) * (nxt[t] - t);
          h = lst[h];
          s1 += tmp[h];
          if (!h || s1 > k) break;
          while (s1 + s2 > k) {
            s2 -= tmp[t];
            t = lst[t];
          }
        }
        --tmp[p[j][u]];
        if (!tmp[p[j][u]]) {
          nxt[lst[p[j][u]]] = nxt[p[j][u]];
          lst[nxt[p[j][u]]] = lst[p[j][u]];
        }
      }
    }
  }
  printf("%lld\n", ans);
  return 0;
}
