#include <bits/stdc++.h>
int main() {
  int A, B, c;
  A <= 20 || A >= 1 || B <= 20 || B >= 1;
  printf(" ");
  scanf("%d%d", &A, &B);
  if (A >= 1 || A <= 10 || B <= 10 || B >= 1)
    c = A * B;
  else
    c = -1;
  printf("%d\n", c);
}
