打表枚举

import java.util.*;

public class Main {

    static int p, q;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        p = sc.nextInt(); q = sc.nextInt();

        int res = 0;
        for (int i = 1; i <= 1000; i++) {
            if (!dfs(i, p, q)) res = i;
        }
        System.out.println(res);
    }

    /**
    * 返回m是否可以被p和q拼凑起来
    */
    static boolean dfs(int m, int p, int q) {
        if (m == 0) return true;
        if (m >= p && dfs(m - p, p, q)) return true;
        if (m >= q && dfs(m - q, p, q)) return true;
        return false;
    }
}

使用公式

a和b最大不能凑出来的整数为m = (a-1) * (b - 1) - 1 = ab - a - b

import java.util.*;

public class Main {

    static int p, q;

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        p = sc.nextInt(); q = sc.nextInt();

        System.out.println((p - 1) * (q - 1) - 1);
    }
}