this question is solved by using Java . if you dont write its code,please write mathematical idea behind of it ?

Problem: Given 100 stones, two players alternate to take stones out. One can take any number from 1 to 15; however, one cannot take any number that was already taken. If in the end of the game, there is k stones left, but 1 - k have all been previously taken, one can take k stones. The one who takes the last stone wins. How can the first player always win