Thus, geographical latitude acting independently of institutions is an important geographic factor affecting power, prosperity, and poverty. The other important geographic factor is whether an area is accessible to ocean-going ships because it lies either on the sea coast or on a navigable river. It costs roughly seven times more to ship a ton of cargo by land than by sea. That puts landlocked countries at an economic disadvantage, and helps explain why landlocked Bolivia and semilandlocked Paraguay are the poorest countries of South America. It also helps explain why Africa, with no river navigable to the sea for hundreds of miles except the Nile, and with fifteen landlocked nations, is the poorest continent. Eleven of those fifteen landlocked African nations have average incomes of $600 or less; only two countries outside Africa (Afghanistan and Nepal, both also landlocked) are as poor.