1999 subtracts its 12, then subtracts the remaining 13, then subtracts the remaining 14, and so on, until finally subtracts the remaining 1 / 1999, finds the last remaining number

1999 subtracts its 12, then subtracts the remaining 13, then subtracts the remaining 14, and so on, until finally subtracts the remaining 1 / 1999, finds the last remaining number

According to the meaning of the title: 1999 × (1-12) × (1-13) × (1-14) × (1-11999) = 1999 × 12 × 23 × 34 × ×19981999=1999×11999=1.