If both numbers are >0, the sum is positive.
If both numbers are <0, the sum is negative.
If there is one + and one - sign, then
search the number with the bigger magnitude, i.e. choose the bigger number irrespective of the sign. The sign of the number with the bigger magnitude is also the sign of the sum.
4+5=? both positive, sum is positive
-4 -6 =? both negative, sum is negative
231 - 521 =?
+521 is bigger than +231, and since 521 is negative, so the sum is also negative. (=-290)
-341 + 425 = ?
425>341, but 425 is positive, so the sum is also positive (=+84)