79(+1)

For each positive integer $n$ , $S(n)$ is the sum of the digits of $n$ . Positive integer $a$ is an extra number, if $\gcd \big(S(a), S(a+1)\big)=k$ , where $k>1$ . For example 79 is an extra number, because $\gcd \big(S(79), S(80)\big)=\gcd(16, 8)=8$ .

If $b$ is the smallest extra number, where $\gcd(S(b), S(b+1))=10$ , then what is $S(b)$ ?