// [문제 링크]: https://school.programmers.co.kr/learn/courses/30/lessons/133502

class Solution {
    public int solution(int[] ingredient) {
        int[] stack = new int[ingredient.length];
        int sp = 0;
        int answer = 0;
        for (int i : ingredient) {
            stack[sp++] = i;
            if (sp >= 4 && stack[sp - 1] == 1
                && stack[sp - 2] == 3
                && stack[sp - 3] == 2
                && stack[sp - 4] == 1) {
                sp -= 4;
                answer++;
            }
        }
        return answer;
    }
}