const func = (s, a, b) => {
let match_empty = /^$/
if (s.match(match_empty)) {
return -1
} else {
let i = s.length - 1
let aIndex = -1
let bIndex = -1
while (aIndex == -1 && bIndex == -1 && i >= 0) {
if (s.substring(i, i + 1) == a) aIndex = i
if (s.substring(i, i + 1) == b) bIndex = i
i--
}
if (aIndex != -1) {
if (bIndex == -1) return aIndex
else return Math.max(aIndex, bIndex)
} else {
if (bIndex != -1) return bIndex
else return -1
}
}
}