How does Zeno end up marrying Augusta in the novel, Confessions of Zeno?

At first, Zeno is determined to marry Ada, the daughter of his friend Giovanni Malfenti, so he starts going over to the Malfenti house every day. When Ada rejects Zeno's proposal, he proposes to her younger sister Alberta, who also rejects him. Afterward, Zeno explains the situation to Augusta, Ada's ugly sister, who agrees to marry him.