I am assuming here you are trying to work with mixed numbers. 

The right way to type the expression 

into 'Alegbrator' is to type:

(1 + 3/4) - (2 + 1/8)

Now the result we get is: -3/8

click here to see the step by step solution

Note:

How do I enter a mixed number (e.g. 5 and 1/2) ?
There is no special notation for mixed numbers. You need to enter them as a sum (e.g. 5+1/2). If the mixed number is negative, make sure to place it in parentheses, e.g. – (5+1/2).