PREPARATIONS are under way in Pyongyang for a rare congress of the Korean Workers' Party which rules North Korea, the first to be held in 36 years, on May 6th. It is something of a coming-out party for Kim Jong Un, its young dictator, who succeeded his father in 2011 and early on promised prosperity to his people, as well as leisure for its young. South of the border, President Park Geun-hye has been appealing to South Korean youth with the idea of unification as a "bonanza"; seventy years on from the peninsula's division, most are disinterested in the idea. For the North, whose minuscule economy is roughly 40 times smaller than that of the South and is only beginning to show signs of reform, that would certainly be the case.

But what of South Korea’s gains? The costs of reunion will be staggering—by conservative estimates about $1 trillion, or three-quarters of annual GDP. Its social-security system would need to provide for 25m Northerners, many of them brutalised and malnourished, and including tens of thousands of prisoners in the North’s gulag. Yet the South would also merge with a population that is younger and has almost twice as many babies. That would be a demographic boon, as South Korea's working-age population begins to shrink from 2017. Disbanding the North’s standing army, the fourth-largest in the world, would free up workers. In total, about 17m workers would join the South’s 36m—though admittedly with far lower skills and education. South Korea would also reap a windfall in reserves of rare earths, which are used in electronics. An estimate from 2012 by a South Korean research institute values the North’s mineral wealth at $10 trillion, 20-odd times larger than that of the South.

Under Japan's occupation from 1910 to 1945, Korea's industrial heartland lay in the north, and therefore North Korea has more railroads than the South. But they are decrepit, and unlike the South’s many sleek motorways, the North’s roads are dire: just 3% are paved. Under the Japanese, Pyongyang was also a thriving centre of Protestant Christianity. Indeed the parents of Kim Il Sung, North Korea's first dictator, were devout Christians. Kim had other ideas, and religious persecution continues under his grandson, Kim Jong Un, the current leader of North Korea; its four churches (two Protestant, one Catholic and one Russian Orthodox) are all in Pyongyang and are for show. A unified Korea would gain another unusual showpiece from the destitute North: the unfinished Ryugyong Hotel, which at 314 metres is the tallest skyscraper on the peninsula—though two new towers currently under construction in the South will both rise above 500 metres within a few years.