function indexEqualsValue(a, beginIndex = 0) {
  let low = 0
  let high = a.length - 1

  while (low <= high) {
    const mid = Math.ceil((low + high) / 2)
    if (mid === a[mid]) {
      const preMid = indexEqualsValue(a.slice(low, mid))
      
      if (preMid !== -1) {
        return low + preMid
      }

      return mid
    }
    
    if (mid > a[mid]) {
      high = mid - 1
    }
    else {
      low = mid + 1
    }
  }

  return -1
}