[OpenJudge-NOI]神奇序列求和

代碼

#include <cstdio>
#include <iostream>
#include <cstring>
using namespace std;

int x,y,n,ans(0);

int main() {
    cin>>x>>y>>n;
    ans=x+y;
    for(int i=1;i<=n;i++) ans=ans*3-(x+y);
    cout<<ans<<endl;
}