#include <bits/stdc++.h>
int main() {
  int a, b;
  scanf("%d%d", &a, &b);
  int c;
  c = a * b;
  if (a <= 9 && a >= 0) {
    if (b <= 9 && b >= 0)
      printf("%d", c);
    else
      printf("-1");
  } else
    printf("-1");
  return 0;
}
