BZOJ原题链接）这题是可以推出公式的：F[i]=F[i-1]*3-F[i-2]+2。套个高精度就好了。（由于需要用到的知识太复杂，推不来……）（其实可以写个暴力推出来）

#include
#include
#include
#include
using namespace std;
const int maxn=105,maxlen=105,tt=10000;
int n;
struct Bigint{
int len,a[maxlen];
Bigint(){len=0;memset(a,0,sizeof(a));}
Bigint operator =(int b){
while (b) a[++len]=b%tt,b/=tt;
}
Bigint operator +(Bigint b){
Bigint c;memset(c.a,0,sizeof(c.a));
c.len=max(len,b.len);
for (int i=1;i<=c.len;i++){
c.a[i]+=a[i]+b.a[i];
c.a[i+1]+=c.a[i]/tt;
c.a[i]%=tt;
}
if (c.a[c.len+1]) c.len++;
return c;
}
Bigint operator -(Bigint b){
Bigint c;memset(c.a,0,sizeof(c.a));
c.len=len;
for (int i=1;i<=c.len;i++){
c.a[i]+=a[i]-b.a[i]+tt;
c.a[i+1]+=c.a[i]/tt-1;
c.a[i]%=tt;
}
if (!c.a[c.len]) c.len--;
return c;
}
Bigint operator *(int b){
Bigint c;memset(c.a,0,sizeof(c.a));
c.len=len;
for (int i=1;i<=c.len;i++){
c.a[i]+=a[i]*b;
c.a[i+1]+=c.a[i]/tt;
c.a[i]%=tt;
}
while (c.a[c.len+1]) c.len++;
return c;
}
void write(){
printf("%d",a[len]);
for (int i=len-1;i>=1;i--) printf("%04d",a[i]);
}
}f[4];
int main(){
scanf("%d",&n);
Bigint two;two=2;
f[1]=1;f[2]=5;
for (int i=3;i<=n;i++) f[i%4]=f[(i-1+4)%4]*3-f[(i-2+4)%4]+two;
f[n%4].write();printf("\n");
return 0;
}