stick to the letter of the law

To follow rules or laws exactly as they are written, as opposed to ignoring parts of them or exercising a more liberal interpretation of them. I know he's the mayor's cousin, but we need to stick to the letter of the law so as to not get accused of giving him preferential treatment. It's frustrating that we pay more in taxes because we stick to the letter of the law, while major companies that flout the regulations get away with paying next to nothing!