E. Two Round Dances

先贴代码题解明天写

#include<bits/stdc++.h> #define int long long using namespace std; signed main() { cin.tie(0);std::ios::sync_with_stdio(false); int n;cin>>n; int fac=1; for(int i=1;i<=n;i++){ fac=fac*i; } fac*=2; fac/=(n*n); cout<<fac<<endl; return 0; }