Sally sharpens knives in her spare time for extra income. Buyers of her service are willing to pay $10 per knife for as many knives as Sally is willing to sharpen. On a particular day, she is willing to sharpen the first knife for $17, the second knife for $7, the third knife for $3, and the fourth knife for $1. Assume Sally is rational in deciding how many knives to sharpen. Her producer surplus is