题目链接：https://www.luogu.com.cn/problem/P3951
结论比较好猜，具体证明：
https://www.luogu.com.cn/blog/Sky-crystal/solution-p3951
AC代码：
1 #include<cstdio> 2 typedef long long ll; 3 int main(){ 4 ll a,b; 5 scanf("%lld%lld",&a,&b); 6 printf("%lld ",a*b-a-b); 7 return 0; 8 9 }