快速切题 sgu123. The sum

123. The sum

time limit per test: 0.25 sec. 
memory limit per test: 4096 KB

 

The Fibonacci sequence of numbers is known: F₁ = 1; F₂ = 1; F_n+1 = F_n + F_n-1, for n>1. You have to find S - the sum of the first K Fibonacci numbers.

 

Input

First line contains natural number K (0<K<41).

 

Output

First line should contain number S.

 

Sample Input

5

 

Sample Output

12

#include <cstdio>
#include <cstring>
using namespace std;
const int maxn=41;
long long f[maxn];
int main(){
    int k;long long s=1;
    scanf("%d",&k);f[1]=1;
    for(int i=2;i<=k;i++){
        f[i]=f[i-1]+f[i-2];
        s+=f[i];
    }
    printf("%I64d\n",s);
    return 0;
}