#include <bits/stdc++.h>
using namespace std;
int read() {
  int x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
  return x * f;
}
int f[15][15], n, m, N, k, ans;
int main() {
  n = read(), m = read(), N = read(), k = read();
  for (int i = 1; i <= N; ++i) {
    int x = read(), y = read();
    f[x][y] = 1;
  }
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j)
      f[i][j] = f[i - 1][j] + f[i][j - 1] - f[i - 1][j - 1] + f[i][j];
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= m; ++j)
      for (int ii = i; ii <= n; ++ii)
        for (int jj = j; jj <= m; ++jj)
          if (f[ii][jj] - f[i - 1][jj] - f[ii][j - 1] + f[i - 1][j - 1] >= k)
            ++ans;
  cout << ans;
  return 0;
}
