CodeForces 630H Benches (组合计数)

题目链接:http://codeforces.com/problemset/problem/630/H

#include<bits/stdc++.h>
using namespace std;

#define debug puts("YES");
#define rep(x,y,z) for(int (x)=(y);(x)<(z);(x)++)
#define ll long long

#define lrt int l,int r,int rt
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define root l,r,rt
#define mst(a,b) memset((a),(b),sizeof(a))
#define pii pair<int,int>
#define fi first
#define se second
#define mk(x,y) make_pair(x,y)
const int mod=1e9+7;
const int maxn=1e5+100;
const int ub=1e6;
ll powmod(ll x,ll y){ll t; for(t=1;y;y>>=1,x=x*x) if(y&1) t=t*x; return t;}
ll gcd(ll x,ll y){
    if(y==0) return x;
    return gcd(y,x%y);
}
ll n,ret=0,ans=0;
int main(){
    cin>>n;n--;
    rep(i,1,n+1){
        ret=0;
        if(i==1){
            ret=4;
            if(i+1<=n) ret=ret*3*powmod(4,n-2);
        }else if(i==n){
            ret=4LL*3*powmod(4,n-2);   
        }else{
            ret=4LL*3*3*powmod(4,n-3);
        }
        ans+=ret;
    }
    cout<<ans<<"\n";
    return 0;
}