#include <bits/stdc++.h>
using namespace std;
template <class K>
inline bool cmax(K& a, const K& b) {
  return (a < b) ? a = b, 1 : 0;
}
template <class K>
inline bool cmin(K& a, const K& b) {
  return (a > b) ? a = b, 1 : 0;
}
inline int read() {
  register int s = 0;
  register char c = getchar();
  while (!isdigit(c)) c = getchar();
  while (isdigit(c)) s = (s << 1) + (s << 3) + c - '0', c = getchar();
  return s;
}
int main() {
  register int a, b;
  a = read(), b = read();
  if (a <= 10 && b <= 10)
    printf("%d", a * b);
  else
    printf("-1");
  return 0;
}
