I Sell Bottled Water for Gold in Another World! Novel

Alex Carter, a typical college graduate, is shocked to discover that his security guard uniform has the ability to transport him to the past — a past that is devastated by famine, where food and water are highly valued commodities. A single bottle of mineral water from today's world can be traded for gold. Two cans of instant meal packs from today's world can buy him one oiran's favor. Moving between the two world, Alex exchanges mundane supplies for wealth, accumulating riches and power in small increments. These days, becoming the richest person in the world in a year's time — well, that is not such an ask, is it? Back in the old days, consolidating the whole empire in two years — completely normal, isn't it?