#include <bits/stdc++.h>
using namespace std;
long a[100][100];
int main() {
  long r, c, n, k, x, y, res = 0;
  scanf("%ld%ld%ld%ld", &r, &c, &n, &k);
  for (long i = 1; i <= n; ++i) scanf("%ld%ld", &x, &y), a[x][y] = 1;
  for (long i = 1; i <= r; ++i)
    for (long j = 1; j <= c; ++j)
      for (long ii = i; ii <= r; ++ii)
        for (long jj = j; jj <= c; ++jj) {
          long cnt = 0;
          for (long iii = i; iii <= ii; ++iii)
            for (long jjj = j; jjj <= jj; ++jjj)
              if (a[iii][jjj]) ++cnt;
          if (cnt >= k) ++res;
        }
  printf("%ld", res);
  return 0;
}
