 <p>In 4 bit 2'scomplement -2 is 1110 and +3 is 0011 so</p> <pre><code>11110 carry 1110 -2 +0011 +3 ---- 10001 which is 0001 or simply 1 ignoring the carry in bit 5 </code></pre> <p>Stepping through the process from the right to the left:</p> <ol> <li><p>1 + 0 results in 1 with no carry</p></li> <li><p>1 + 1 results in 0 with a carry of 1</p></li> <li><p>1 + 0 + the carry of 1 results 0 with a carry of 1</p></li> <li><p>1 + 0 + the carry of 1 results in 0 with a carry of 1</p></li> <li><p>just the carry of 1 results in 1 with nothing more to carry</p></li> </ol> <p>For reference see the Wikipedia article on 2's complement particularly the section on addition at <a href="https://en.wikipedia.org/wiki/Two%27s_complement#Addition" rel="nofollow">https://en.wikipedia.org/wiki/Two%27s_complement#Addition</a>. There are a number of online 2's complement calculators to help with conversions and one of them is at <a href="http://www.exploringbinary.com/twos-complement-converter/" rel="nofollow">http://www.exploringbinary.com/twos-complement-converter/</a></p> <p>Let me know if you want to see how -3 + -3 is done, since that is what you attempted. It is a similar process, but be sure start with bit length large enough to avoid overflow as determined when the leftmost two bits in the carry row have different values.</p>