#include <bits/stdc++.h>
int main() {
  int a, b, c;
  scanf("%d%d", &a, &b);
  if (a < 0 || a > 10 || b < 0 || b > 10) {
    printf("-1\n");
  } else {
    c = a * b;
    printf("%d", c);
  }
  return 0;
}
