#JavaScript (ES6), <s>50</s> <s>39</s> 38 bytes <!-- language: lang-js --> x=s=`` f=n=>-~s.search(n)||f(n,s+=++x) --- ##Try it <!-- begin snippet: js hide: false console: true babel: false --> <!-- language: lang-js --> x=s=`` f=n=>-~s.search(n)||f(n,s+=++x) o.innerText=f(i.value=15);oninput=_=>o.innerText=f(+i.value) <!-- language: lang-html --> <input id=i type=number><pre id=o></pre> <!-- end snippet -->