Paper 1 October/November 2003 Economics (9708/03) A Level

In the diagram, ${S_1}{S_1}$ and DD represent the original supply and demand curves for an agricultural product.
Bad weather then reduces supply to ${S_2}{S_2}$.
The government does not allow the price to rise above OP1.
How much of the product will the government have to supply from a buffer stock if demand is to be met?