Keiyakukon shita Aite ga Oni Saishou deshita ga, Kono Tabi Saishoushitsu Sennin Hosakan ni Ninmei sareta Jimi Bunkan (Hensouchuu) wa Watashi desu.

After the reigning king married a commoner, marriage for love has been all the rage in the kingdom where Christine lives. Even if one is already married, it holds no sway in the face of "true love" or at least that has become the custom. Just as Christine is feeling fed up with these marriage values, she meets Leon. To get back at her serial-cheater of a fiancé, she and Leon pretend to have found true love with each other and break off Christine's engagement. It comes as quite a shock when she finds out he is actually the chancellor...!

(Source: Kodansha)