#include <bits/stdc++.h>
double const EPS = 3e-8;
using namespace std;
template <class T>
T gcd(T a, T b) {
  return (b != 0 ? gcd<T>(b, a % b) : a);
}
template <class T>
T lcm(T a, T b) {
  return (a / gcd<T>(a, b) * b);
}
struct debugger {
  template <typename T>
  debugger& operator,(const T& v) {
    cerr << v << "\t";
    return *this;
  }
} dbg;
template <class A, class B>
ostream& operator<<(ostream& o, const pair<A, B>& p) {
  return o << "(" << p.first << ", " << p.second << ")";
}
template <class T>
ostream& operator<<(ostream& o, const vector<T>& v) {
  o << "[";
  for (__typeof((v).end()) it = (v).begin(); it != (v).end(); ++it)
    o << *it << ", ";
  return o << "]";
}
template <class T>
ostream& operator<<(ostream& o, const set<T>& v) {
  o << "[";
  for (__typeof((v).end()) it = (v).begin(); it != (v).end(); ++it)
    o << *it << ", ";
  return o << "]";
}
template <class T>
inline bool read(T& x) {
  int c = getchar();
  int sgn = 1;
  while (~c && c < '0' || c > '9') {
    if (c == '-') sgn = -1;
    c = getchar();
  }
  for (x = 0; ~c && '0' <= c && c <= '9'; c = getchar()) x = x * 10 + c - '0';
  x *= sgn;
  return ~c;
}
const int NX = 12;
int inp[NX][NX], r, c, n, k;
int rectangle(int x1, int y1, int x2, int y2) {
  int tot = 0;
  for (int i = x1; i <= x2; i++) {
    for (int j = y1; j <= y2; j++) tot += inp[i][j];
  }
  return tot >= k;
}
int main() {
  r = ({
    int a;
    read(a);
    a;
  }),
  c = ({
    int a;
    read(a);
    a;
  }),
  n = ({
    int a;
    read(a);
    a;
  }),
  k = ({
    int a;
    read(a);
    a;
  });
  for (__typeof((n)-1) i = (0); i <= (n)-1; ++i) {
    int x = ({
      int a;
      read(a);
      a;
    }),
        y = ({
          int a;
          read(a);
          a;
        });
    inp[x][y] = 1;
  }
  int ans = 0;
  for (int i = 1; i <= r; i++) {
    for (int j = 1; j <= c; j++) {
      for (int x = i; x <= r; x++) {
        for (int y = j; y <= c; y++) {
          ans += rectangle(i, j, x, y);
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
