やいなや: the minute (that) ...; no sooner than ...; whether or not

誤あやまりに気きづくやいなや方針ほうしんを改あらためた部長ぶちょうは、まさに君子豹変くんしひょうへんの人ひとだ。

The department head, who changed policy the moment he saw his error, is truly one who adapts like a wise man.