gcd of negative numbers

Since $\gcd$ means greatest common divisor, if we have $\gcd(a,b)=13$ , then $\gcd(-a,-b)$ will be $\boxed{13}$ only because $13$ is greater than all negative numbers.

For example if $a=13$ and $b=26$ , then we have $\gcd(a,b)=13$ and also $\gcd(-13,-26)=13$ .