function Main(input) {
  lines = input.split('\n')
  line = lines[0].split(' ')
  n = parseInt(line[0])
  m = parseInt(line[1])
  if (n < 10 && m < 10) {
    console.log(n * m)
  } else {
    console.log(-1)
  }
}

Main(require('fs').readFileSync('/dev/stdin', 'utf8'))