#include <bits/stdc++.h>
bool V[11][11];
int T[11][11];
int R = 0, C = 0, N = 0, K = 0, t1 = 0, t2 = 0;
int W = 0;
int main() {
  scanf("%d%d%d%d", &R, &C, &N, &K);
  for (int i = 1; i <= N; i++) {
    scanf("%d%d", &t1, &t2);
    V[t1][t2] = true;
  }
  for (int i = 1; i <= R; i++)
    for (int j = 1; j <= C; j++) {
      T[i][j] = T[i - 1][j] + T[i][j - 1] - T[i - 1][j - 1];
      if (V[i][j]) T[i][j]++;
    }
  for (int i = 1; i <= R; i++)
    for (int j = 1; j <= C; j++)
      for (int k = i; k <= R; k++)
        for (int l = j; l <= C; l++) {
          if (T[k][l] - T[i - 1][l] - T[k][j - 1] + T[i - 1][j - 1] >= K) W++;
        }
  printf("%d\n", W);
  return 0;
}
