From any one vertex of a 4-sided polygon (a quadrilateral), 1 diagonal can be drawn. From any one vertex of a 5-sided polygon (a pentagon), 2 diagonals can be drawn. From any one vertex of a 6-sided polygon (a hexagon), 3 diagonals can be drawn. From any one vertex of a 7-sided polygon (a heptagon), 4 diagonals can be drawn. The number of diagonals that can be drawn from any one vertex of a 20-sided polygon (a icosagon) is .