This problem is in Contest(s) below:
Contest 2758:入门-第2课-回家作业(雷老师)
Contest 2992:(AK张老师)入门班
Contest 2994:入门-第2课-课堂练习(王老师)
Contest 2759:入门-第2课-课堂练习(雷老师)
Contest 2987:入门-第2课-课堂练习(崔)
Contest 2993:(AK张老师)入门二班 运算