How Can Penis Size Be Increased?

Hello Doctor Robert Galamaga I Am a 19 year old male suffering from a small penis, when it is erected it is about 8- 9 inched long but when it is not erected it goes to about 1.5 - 2 inches long my best mate seen my penis and said it was small and his penis is about 4 inches not erected and he is younger than me i want to get bigger when im not erected so im not ashamed of what i have thanks