#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int ret = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -f;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') ret = ret * 10 + ch - '0', ch = getchar();
  return ret * f;
}
int n, r, c, k;
bool flag[15][15];
int main() {
  r = read(), c = read(), n = read(), k = read();
  for (int i = 1; i <= n; i++) {
    int x = read(), y = read();
    flag[x][y] = 1;
  }
  int sum = 0;
  for (int i1 = 1; i1 <= r; i1++)
    for (int j1 = 1; j1 <= c; j1++)
      for (int i2 = i1; i2 <= r; i2++)
        for (int j2 = j1; j2 <= c; j2++) {
          int ans = 0;
          for (int x = i1; x <= i2; x++)
            for (int y = j1; y <= j2; y++)
              if (flag[x][y]) ans++;
          if (ans >= k) sum++;
        }
  cout << sum;
  return 0;
}
