I can think of it this way, without changing the sign of 1, and the operation (subtraction).
Subtract the opposite of the absolute value of 1 from 19.
Opposite of the absolute value of 1 = - I 1 I
Equation:
19 - (- I 1 I) = 20
19 - (- 1) = 20
19 + 1 = 20 (When subtracting integers, change the sign of the
subtrahend, then proceed to addition rule.)
20 = 20
