其它
There are two groups of balls. Group $A$ has $50$ balls and Group $B$ has $40$ balls. Candy and Nini want to play a game with these balls. They will take turns to take balls from the two groups. Each person can take any number of balls from a group at a time, but they should take at least one ball at a time. The person who takes the last ball in the two groups will win the game. If Nini starts the game first, how many balls should she take to guarantee her success? ． 
Overseas Competition->Knowledge Point->Combinatorics->Strategies and Operations->Game Strategy
Nini needs to take $$50-40=10$$ balls in Group A which will make the two groups have the same amount of balls. Then, no matter how many balls Candy takes in a group, Nini will take as many balls as Candy took before in the other group. 