The site of the current bridge is among several sites of a slave rebellion called the Stono Rebellion, one of the earliest known organized acts against slavery in the Americas.  On September 9, 1739, twenty black slaves raided a store near the bridge; in the process they killed two storekeepers and took guns and gunpowder. The bridge survived an attempt by Union forces to burn it during the American Civil War. Union troops floated burning rafts down to the Stono Bridge, hoping the wooden structure would catch fire and burn. However, their efforts were thwarted by a Lieutenant Smith, who along with members of a naval battalion, brought the rafts to shore. There have been several bridges in this location; the last variation was a two lane bridge 1,416 feet -long, which was built in 1928, and later refurbished in 1950. This swing-span drawbridge used a 156-foot -long steel-truss which left little room for even small vessels to pass beneath unless it was open.
Question: How many more slaves raided the store than storekeepers were killed?
Answer: 18