Integers are positive whole numbers, negative whole numbers, and zero (no fractions or decimals).

When adding two integers with the same sign (either positive or negative), add the numbers and keep the sign:

4 + 5 = 9

-8 + -9 = -17

When adding integers with different signs, subtract the numbers and keep the sign from the "larger" one (that is, the number that is larger if you disregard the positive or negative sign).

5 + -9 = -4