sync music and delete. iphone5

I have 255 songs on my imac desktop. i wanted to move thoose songs to my iphone5. when i did that it asked me if i wanted to erase and put the 255 songs on my iphone. i said yes. it delted all my songs but only put 18 out of 255 songs on my iphone. how do i get all 255 songs to my iphone5?