The answer is that from 1 to 9, it is impossible to calculate 26 by adding and subtracting.

This is my evidence:

1. If the sum of two integers is even, then the parity of these two numbers is the same.

2. Because (A+B)+(A-B)=2A is an even number, it can be inferred from the conclusion 1 that the parity of A+B and a-b is the same (both a and b are integers).

3. 1+2+3+4+5+6+7+8+9=45 is an odd number, so it can be deduced from conclusion 2 that the number obtained by arbitrarily adding or subtracting the nine numbers (each number is used only once) is the same as 45 and can only be an odd number. So it is impossible to calculate 26 by adding and subtracting 1 to 9.