Luke grows carrots in his garden, with the following private supply curve for kilos of carrots P = 29.9 + 8.2Q. Every additional kilo of carrots requires some smelly cow manure, which increases the value by 12.2$. However the manure required by each additional kilo of carrots also causes discomfort for the neighbours, imposing a cost of 12.2$ on them. What is the slope of the social supply curve?