#include <bits/stdc++.h>
const int maxn = 11;
using namespace std;
int XYlen[maxn][maxn][maxn][maxn], ans;
int main() {
  int x, y, r, c, n, k;
  scanf("%d%d%d%d", &r, &c, &n, &k);
  for (int i = 1; i <= n; i++) {
    scanf("%d%d", &x, &y);
    for (int i = 1; i <= x; i++)
      for (int o = 1; o <= y; o++)
        for (int j = x; j <= r; j++)
          for (int z = y; z <= c; z++) XYlen[i][j][o][z]++;
  }
  for (x = 1; x <= r; x++)
    for (y = 1; y <= c; y++)
      for (int nx = x; nx <= r; nx++)
        for (int ny = y; ny <= c; ny++)
          if (XYlen[x][nx][y][ny] >= k) ans++;
  printf("%d", ans);
  return 0;
}
