题解 | #跳石板#

import java.util.*;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int N = in.nextInt();
        int M = in.nextInt();
        int[] ret = new int[M+1];

        ArrayList<Integer> arr = new ArrayList<>();

        for (int i = N+1; i <= M; i++) {
            ret[i] = Integer.MAX_VALUE;
        }

        for (int i = N; i < M; i++) {
            if (ret[i] == Integer.MAX_VALUE) {
                continue;
            }
            arr = new ArrayList<>();
            abc(i, arr);
            // for (int j = 0; j < arr.size(); j++) {
            //     int tmp = arr.get(j);
            //     if (i + tmp < M && ret[i + tmp] == 0) {
            //         ret[i + tmp] = ret[i] + 1;
            //     }else if(i + tmp == M) {
            //         ret[M] = ret[i] + 1;
            //     }
            // }
            for (int tmp : arr) {
                if (i + tmp <= M && ret[i + tmp] != Integer.MAX_VALUE) {
                    ret[i + tmp] = Math.min(ret[i] + 1, ret[i + tmp]);
                }else if(i + tmp <= M) {
                    ret[i + tmp] = ret[i] + 1;
                }
            }
        }
        if (ret[M] == Integer.MAX_VALUE) {
            System.out.println(-1);
        }else {
            System.out.println(ret[M]);
        }
    }

    public static void abc(int x, ArrayList<Integer> arr) {
        for (int i = 2; i * i <= x; i++) {
            if (x % i == 0) {
                arr.add(i);
                if (x / i != i) {
                    arr.add(x/i);
                }
            }
        }
    }
}