A tourist purchased a total of 1500 dollars worth of traveler's checks in 10 dollars and 50 dollars denominations. During the trip the tourist cashed 7 checks and then lost all of the rest. If the number of 10 dollar checks cashed was one more or one less than the number of 50 dollar checks, what is the minimum possible value of the checks that were lost?

1500 - 70 = 1430

1430 - 50 = 1380

1380 - 70 = 1310

My answer is 1310 dollars.

The book's answer is 1270 dollars.

I got lost in the application in terms of the word OR in the following statement:

If the number of 10 dollar checks cashed was one more or one less than the number of 50 dollar checks, what is the minimum possible value of the checks that were lost?

Why is my answer wrong? Can someone set up an equation for me to solve?