function main(input) {
    console.log(input);
  input = input.split(" ");
  if((input[0] < 1 || input[0] > 20) && 
    (input[1] < 1 || input[1] > 20) ) {
  		console.log(-1);
  }
  console.log(input[0] * input[1]);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));