Language: C++

/***********************************************************
 *This code By @1353055672(Ligen)                          *
 *[Warning]You're not excepted to understand this code!    *
 *CSP-S 2019 RP++                                          *
 ***********************************************************/
// #pragma GCC optimize(3)
#define ADD_STACK int size = 512 << 20;\
	char *pp = (char*)malloc(size) + size;  \
	__asm__("movl %0, %%esp\n" :: "r"(pp))
#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<string>
#include<fstream>
#include<queue>
#include<map>
#include<stack>
#include<set>
#include<bitset>
#include<ctime>
#include<vector>
#define ll long long
#define ull unsigned long long
#define mn 200020
#define Max(x,y) (((x)>(y))?(x):(y))
#define Min(x,y) (((x)<(y))?(x):(y))
#define Abs(x) (((x)<(0))?(-(x)):(x))
#define infll (ll)(1e18)
#define infint (1<<30)
#define mod (int)(1e9+7)
#define FOR(a,b,c) for (register int a=b;a<=c;++a)
#define FORD(a,b,c) for (register int a=b;a>=c;--a)
using namespace std;
// char buf[1<<20],*p1=buf,*p2=buf;
// #define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<20,stdin),p1==p2)?EOF:*p1++)
inline ll read(){
	ll x=0,f=1;char c;
	for(c=getchar();c<'0'||c>'9';f=((c=='-')?-1:f),c=getchar());
	for(;c>='0'&&c<='9';x=x*10+c-'0',c=getchar());
	return x*f;
}
template<typename T>
inline void write(T a){
	if(a==0){putchar('0');return;}if(a<0)putchar('-'),a=-a;char c1[120];int h=0;
	while(a)c1[++h]=a%10+'0',a/=10;FORD(i,h,1)putchar(c1[i]);
}
inline void write_(){return;}
template<typename T,typename... Args>
inline void write_(T a,Args... b){write(a);putchar(' ');write_(b...);}
inline void writeln(){putchar('\n');return;}
template<typename T,typename... Args>
inline void writeln(T a,Args... b){write(a);putchar(' ');writeln(b...);}
//need c++11

//inline void write(ll a){
//    if(a==0){putchar('0');return;}if(a<0)putchar('-'),a=-a;char c1[120];int h=0;
//    while(a)c1[++h]=a%10+'0',a/=10;FORD(i,h,1)putchar(c1[i]);
//}
//inline void write_(ll a){write(a);putchar(' ');}
//inline void writeln(ll a){write(a);putchar('\n');}
inline ll gcd(ll a,ll b){return a==0?b:gcd(b%a,a);}
inline ll lcm(ll a,ll b){return 1ll*a/gcd(a,b)*b;}
inline ll Pow(ll n,ll a){ll b=1;while(a){if(a&1)b=1ll*b*n%mod;n=1ll*n*n%mod;a>>=1;}return b;}
//---------------------Head Files--------------------------//

signed main(){
#ifdef LOCAL_LIGEN
	freopen("0.in","r",stdin);
	// freopen("0.out","w",stdout);
	double be=clock();
#endif
	int a=read(),b=read();
	if(a>9)return writeln(-1);
	if(b>9)return writeln(-1);
	writeln(a*b);
#ifdef LOCAL_LIGEN
	double en=clock();
	printf("Time: %.0lfms\n",en-be);
	fclose(stdin);fclose(stdout);
#endif
	return 0;
}


Compiler output:
a.cc: In function 'void write(T)':
a.cc:47:45: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
   47 |         while(a)c1[++h]=a%10+'0',a/=10;FORD(i,h,1)putchar(c1[i]);
      |                                             ^
a.cc:34:39: note: in definition of macro 'FORD'
   34 | #define FORD(a,b,c) for (register int a=b;a>=c;--a)
      |                                       ^
a.cc: In function 'int main()':
a.cc:75:30: error: void value not ignored as it ought to be
   75 |         if(a>9)return writeln(-1);
      |                       ~~~~~~~^~~~
a.cc:76:30: error: void value not ignored as it ought to be
   76 |         if(b>9)return writeln(-1);
      |                       ~~~~~~~^~~~
a.cc: In instantiation of 'void write(T) [with T = int]':
a.cc:54:41:   required from 'void writeln(T, Args ...) [with T = int; Args = {}]'
   54 | inline void writeln(T a,Args... b){write(a);putchar(' ');writeln(b...);}
      |                                    ~~~~~^~~
a.cc:75:23:   required from here
   75 |         if(a>9)return writeln(-1);
      |                       ~~~~~~~^~~~
a.cc:47:45: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
   47 |         while(a)c1[++h]=a%10+'0',a/=10;FORD(i,h,1)putchar(c1[i]);
      |                                             ^
a.cc:34:39: note: in definition of macro 'FORD'
   34 | #define FORD(a,b,c) for (register int a=b;a>=c;--a)
      |                                       ^