Paul is at the orchestra. The string section is arranged in an r × c rectangular grid and is filled with violinists with the exception of n violists. Paul really likes violas, so he would like to take a picture including at least k of them. Paul can take a picture of any axis-parallel rectangle in the orchestra. Count the number of possible pictures that Paul can take.

Two pictures are considered to be different if the coordinates of corresponding rectangles are different.

Input

The first line of input contains four space-separated integers r, c, n, k (1 ≤ r, c, n ≤ 3000, 1 ≤ k ≤ min(n, 10)) — the number of rows and columns of the string section, the total number of violas, and the minimum number of violas Paul would like in his photograph, respectively.

The next n lines each contain two integers xi and yi (1 ≤ xi ≤ r, 1 ≤ yi ≤ c): the position of the i-th viola. It is guaranteed that no location appears more than once in the input.

Output

Print a single integer — the number of photographs Paul can take which include at least k violas. 

Examples

Input

2 2 1 1
1 2


Output

4


Input

3 2 3 3
1 1
3 1
2 2


Output

1


Input

3 2 3 2
1 1
3 1
2 2


Output

4

Note

We will use '*' to denote violinists and '#' to denote violists.

In the first sample, the orchestra looks as follows: 
    
    
      
    *#  
    **  
    

Paul can take a photograph of just the viola, the 1 × 2 column containing the viola, the 2 × 1 row containing the viola, or the entire string section, for 4 pictures total.

In the second sample, the orchestra looks as follows: 
    
    
      
    #*  
    *#  
    #*  
    

Paul must take a photograph of the entire section.

In the third sample, the orchestra looks the same as in the second sample.
#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class t, class u>
void chmax(t& first, u second) {
  if (first < second) first = second;
}
template <class t, class u>
void chmin(t& first, u second) {
  if (second < first) first = second;
}
template <class t>
using vc = vector<t>;
template <class t>
using vvc = vc<vc<t>>;
using pi = pair<int, int>;
using vi = vc<int>;
template <class t, class u>
ostream& operator<<(ostream& os, const pair<t, u>& p) {
  return os << "{" << p.first << "," << p.second << "}";
}
template <class t>
ostream& operator<<(ostream& os, const vc<t>& v) {
  os << "{";
  for (auto e : v) os << e << ",";
  return os << "}";
}
using uint = unsigned;
using ull = unsigned long long;
template <int i, class T>
void print_tuple(ostream&, const T&) {}
template <int i, class T, class H, class... Args>
void print_tuple(ostream& os, const T& t) {
  if (i) os << ",";
  os << get<i>(t);
  print_tuple<i + 1, T, Args...>(os, t);
}
template <class... Args>
ostream& operator<<(ostream& os, const tuple<Args...>& t) {
  os << "{";
  print_tuple<0, tuple<Args...>, Args...>(os, t);
  return os << "}";
}
void print(ll x, int suc = 1) {
  cout << x;
  if (suc == 1) cout << "\n";
  if (suc == 2) cout << " ";
}
ll read() {
  ll i;
  cin >> i;
  return i;
}
vi readvi(int n, int off = 0) {
  vi v(n);
  for (int i = int(0); i < int(n); i++) v[i] = read() + off;
  return v;
}
template <class T>
void print(const vector<T>& v, int suc = 1) {
  for (int i = int(0); i < int(v.size()); i++)
    print(v[i], i == int(v.size()) - 1 ? suc : 2);
}
string readString() {
  string s;
  cin >> s;
  return s;
}
template <class T>
T sq(const T& t) {
  return t * t;
}
void yes(bool ex = true) {
  cout << "Yes" << endl;
  if (ex) exit(0);
}
void no(bool ex = true) {
  cout << "No" << endl;
  if (ex) exit(0);
}
constexpr ll ten(int n) { return n == 0 ? 1 : ten(n - 1) * 10; }
const ll infLL = LLONG_MAX / 3;
const int inf = INT_MAX / 2 - 100;
int topbit(signed t) { return t == 0 ? -1 : 31 - __builtin_clz(t); }
int topbit(ll t) { return t == 0 ? -1 : 63 - __builtin_clzll(t); }
int botbit(signed first) { return first == 0 ? 32 : __builtin_ctz(first); }
int botbit(ll first) { return first == 0 ? 64 : __builtin_ctzll(first); }
int popcount(signed t) { return __builtin_popcount(t); }
int popcount(ll t) { return __builtin_popcountll(t); }
bool ispow2(int i) { return i && (i & -i) == i; }
int mask(int i) { return (int(1) << i) - 1; }
bool inc(int first, int second, int c) {
  return first <= second && second <= c;
}
template <class t>
void mkuni(vc<t>& v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}
ll rand_int(ll l, ll r) {
  static random_device rd;
  static mt19937_64 gen(rd());
  return uniform_int_distribution<ll>(l, r)(gen);
}
template <class t>
int lwb(const vc<t>& v, const t& first) {
  return lower_bound(v.begin(), v.end(), first) - v.begin();
}
template <class t>
using V = vc<t>;
template <class t>
using VV = vvc<t>;
int popcnt(uint x) { return __builtin_popcount(x); }
int popcnt(ull x) { return __builtin_popcountll(x); }
int bsr(uint x) { return 31 - __builtin_clz(x); }
int bsr(ull x) { return 63 - __builtin_clzll(x); }
int bsf(uint x) { return __builtin_ctz(x); }
int bsf(ull x) { return __builtin_ctzll(x); }
struct FastSet {
  static constexpr uint B = 64;
  int n, lg;
  VV<ull> seg;
  FastSet(int _n) : n(_n) {
    do {
      seg.push_back(V<ull>((_n + B - 1) / B));
      _n = (_n + B - 1) / B;
    } while (_n > 1);
    lg = seg.size();
  }
  bool operator[](int i) const { return (seg[0][i / B] >> (i % B) & 1) != 0; }
  void set(int i) {
    for (int h = 0; h < lg; h++) {
      seg[h][i / B] |= 1ULL << (i % B);
      i /= B;
    }
  }
  void reset(int i) {
    for (int h = 0; h < lg; h++) {
      seg[h][i / B] &= ~(1ULL << (i % B));
      if (seg[h][i / B]) break;
      i /= B;
    }
  }
  int next(int i) {
    for (int h = 0; h < lg; h++) {
      if (i / B == seg[h].size()) break;
      ull d = seg[h][i / B] >> (i % B);
      if (!d) {
        i = i / B + 1;
        continue;
      }
      i += bsf(d);
      for (int g = h - 1; g >= 0; g--) {
        i *= B;
        i += bsf(seg[g][i / B]);
      }
      return i;
    }
    return n;
  }
  int prev(int i) {
    i--;
    for (int h = 0; h < lg; h++) {
      if (i == -1) break;
      ull d = seg[h][i / B] << (63 - i % 64);
      if (!d) {
        i = i / B - 1;
        continue;
      }
      i += bsr(d) - (B - 1);
      for (int g = h - 1; g >= 0; g--) {
        i *= B;
        i += bsr(seg[g][i / B]);
      }
      return i;
    }
    return -1;
  }
};
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(20);
  int r, c, n, k;
  cin >> r >> c >> n >> k;
  vvc<int> pos(r);
  for (int _ = int(0); _ < int(n); _++) {
    int x, y;
    cin >> x >> y;
    x--;
    y--;
    pos[x].push_back(y);
  }
  ll ans = 0;
  vi p, q;
  for (int st = int(0); st < int(r); st++) {
    FastSet s(c + 2);
    vi raw(c + 2);
    raw[0] = inf;
    raw[c + 1] = inf;
    s.set(0);
    s.set(c + 1);
    int cur = c * (c + 1) / 2;
    auto waf = [&](int i) {
      int first = k - raw[i];
      if (first <= 0) return 0;
      p.clear();
      q.clear();
      {
        int x = i;
        while ((int)p.size() < first) {
          x = s.prev(x);
          int w = min(first - (int)p.size(), raw[x]);
          for (int _ = int(0); _ < int(w); _++) p.push_back(x);
        }
        reverse(p.begin(), p.end());
      }
      p.push_back(i);
      {
        int x = i;
        while ((int)q.size() < first) {
          x = s.next(x + 1);
          int w = min(first - (int)q.size(), raw[x]);
          for (int _ = int(0); _ < int(w); _++) q.push_back(x);
        }
      }
      int res = 0;
      for (int j = int(0); j < int(first); j++) {
        res -= (p[j + 1] - p[j]) * (q[j] - i);
      }
      p.erase(p.begin());
      q.pop_back();
      first--;
      for (int j = int(0); j < int(first); j++) {
        res += (p[j + 1] - p[j]) * (q[j] - i);
      }
      s.set(i);
      raw[i]++;
      return res;
    };
    for (int j = int(st); j < int(r); j++) {
      for (auto z : pos[j]) {
        cur += waf(z + 1);
      }
      ans += c * (c + 1) / 2 - cur;
    }
  }
  cout << ans << endl;
}
