Two teams face a field of 21 flags. The teams take turns choosing 1, 2, or 3 flags at each turn. The team that is able to claim the last flag win. What is your strategy if you are allowed to choose frist

number flags 1~21
assume team1 wants to claim flag21 to win.
team1 must NOT claim flag18
in order not to claim flag18, it has to claim flag17.
in order to claim flag17, it must NOT claim flag14
so, team 1 has to claim flag17, 13, 9, 5 and 1, and not to claim 18, 14, 10, 6, and 2 to win.
anyone disagree? ^^