题目：

 

1 #include 
     
       2 using namespace std; 3 typedef long long ll; 4 const int M = 1e5+5; 5  6 int n; 7 ll a[50]; 8 int main() { 9     a[1] = 2;10     a[2] = 2;11     a[3] = 4;12     for( int i=4; i<=45; i++ )  13         a[i] = a[i-1]+a[i-2];14     while( ~scanf("%d", &n ) )   {15         printf("%I64d\n", a[n] );16     }    17 }