#include <bits/stdc++.h>
using namespace std;
const long long inf = 9e18;
const double pi = acos(-1.0);
const long long M = inf;
const long long N = 20 + 5;
long long power(long long b, long long n) {
  long long ans(1);
  while (n) {
    if (n & 1) ans = (ans * b) % M;
    b = (b * b) % M;
    n >>= 1;
  }
  return ans;
}
long long a[N][N];
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  long long r, c, n, x, co(0);
  cin >> r >> c >> n >> x;
  while (n--) {
    long long x, y;
    cin >> x >> y;
    a[x][y] = 1;
  }
  for (long long i = 1; i <= r; ++i) {
    for (long long j = 1; j <= c; ++j) {
      for (long long k = i; k <= r; ++k) {
        for (long long l = j; l <= c; ++l) {
          long long no(0);
          for (long long m = i; m <= k; ++m) {
            for (long long n = j; n <= l; ++n) {
              no += a[m][n];
            }
          }
          if (no >= x) ++co;
        }
      }
    }
  }
  cout << co;
}
