#include <bits/stdc++.h>
using namespace std;
inline int read() {
  char c = getchar();
  int n = 0;
  bool f = 0;
  while (c < '0' || c > '9') {
    if (c == '-') f = 1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') {
    n = (n << 1) + (n << 3) + (c ^ 48);
    c = getchar();
  }
  return f ? -n : n;
}
int r, c, n, t, x, y, ans, res;
bool m[101][101];
int main() {
  r = read(), c = read(), n = read(), t = read();
  for (int i = 1; i <= n; i++) {
    x = read(), y = read();
    m[x][y] = 1;
  }
  for (int i = 1; i <= r; i++)
    for (int j = 1; j <= c; j++)
      for (int k = i; k <= r; k++)
        for (int h = j; h <= c; h++) {
          res = 0;
          for (int a = i; a <= k; a++)
            for (int b = j; b <= h; b++)
              if (m[a][b]) res++;
          if (res >= t) ans++;
        }
  printf("%d\n", ans);
  return 0;
}
