25. Find the smallest positive integer $n$ such that: If each vertex of a regular $n$-gon is arbitrarily colored with one of the three colors red, yellow, or blue, then among the $n$ vertices, there must exist four vertices of the same color that form the vertices of an isosceles trapezoid (a convex quadrilateral with two parallel sides and two non-parallel sides of equal length is called an isosceles trapezoid).