https://habr.com/en/company/postgrespro/blog/467437/ gives the following example of a Lost Update:
For example, two transactions are going to increase the amount on the same account by ₽100 (₽ is the currency sign for Russian rouble). The first transaction reads the current value (₽1000) and then the second transaction reads the same value. The first transaction increases the amount (this gives ₽1100) and writes this value. The second transaction acts the same way: it gets the same ₽1100 and writes this value. As a result, the customer lost ₽100
I read this a few times. But I don’t understand how the customer lost P100. Please explain.