#include <bits/stdc++.h>
using namespace std;
const int next_x[8] = {0, 1, -1, 0, 1, 1, -1, -1};
const int next_y[8] = {1, 0, 0, -1, 1, -1, -1, 1};
const int inf = 2147483647;
const long long linf = 1e18 + 5;
const double PI = acos(-1.0);
struct IO {
  char buf[(1 << 20)], *p1, *p2;
  char pbuf[(1 << 20)], *pp;
  IO() : p1(buf), p2(buf), pp(pbuf) {}
  ~IO() { fwrite(pbuf, 1, pp - pbuf, stdout); }
  inline char gc() {
    if (p1 == p2) p2 = (p1 = buf) + fread(buf, 1, (1 << 20), stdin);
    return p1 == p2 ? -1 : *p1++;
  }
  inline bool blank(char ch) {
    return ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';
  }
  template <class T>
  inline void read(T &x) {
    register double tmp = 1;
    register bool sign = 0;
    x = 0;
    register char ch = gc();
    for (; !(ch >= '0' && ch <= '9'); ch = gc())
      if (ch == '-') sign = 1;
    for (; (ch >= '0' && ch <= '9'); ch = gc()) x = x * 10 + (ch - '0');
    if (ch == '.')
      for (ch = gc(); (ch >= '0' && ch <= '9'); ch = gc())
        tmp /= 10.0, x += tmp * (ch - '0');
    if (sign) x = -x;
  }
  inline void read(char *s) {
    register char ch = gc();
    for (; blank(ch); ch = gc())
      ;
    for (; !blank(ch); ch = gc()) *s++ = ch;
    *s = 0;
  }
  inline void read(char &c) {
    for (c = gc(); blank(c); c = gc())
      ;
  }
  inline void push(const char &c) {
    if (pp - pbuf == (1 << 20)) fwrite(pbuf, 1, (1 << 20), stdout), pp = pbuf;
    *pp++ = c;
  }
  template <class T>
  inline void write(T x) {
    if (x < 0) x = -x, push('-');
    static T sta[35];
    T top = 0;
    do {
      sta[top++] = x % 10, x /= 10;
    } while (x);
    while (top) push(sta[--top] + '0');
  }
  template <class T>
  inline void write(T x, char lastChar) {
    write(x), push(lastChar);
  }
} io;
const double eps = 1e-10;
const int MAXN = 1e6 + 5;
const long long MAX = 1e10;
int work() {
  int a, b;
  scanf("%d%d", &a, &b);
  if (a <= 9 && b <= 9)
    printf("%d\n", a * b);
  else
    printf("-1\n");
  return 0;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  work();
  return 0;
}
