一回生两回熟/ 一回生兩回熟yīhuíshēngliǎnghuíshú(also yīhuíshēngliǎnghuíshóu)strangers at first, friends the next time
(idiom) strangers at the first meeting, but soon well-acquainted or comfortable with each other after meeting a second time; unfamiliar at first, but easy the second time