SGU - 123 - The sum （简单数学！）

SGU - 123

The sum

Time Limit: 250MS Memory Limit: 4096KB 64bit IO Format: %I64d & %I64u

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Description

Here is your second problem, keep calm and solve it .

Nacci sequence of numbers is known to all : 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 Nacci numbers.

 

Input

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

 

Output

First line should contain number S.

 

Sample Input

5

 

Sample Output

12

Source

简单数学求和题，，居然int都不会爆，，刚开始我还以为要long long。。

AC代码：

#include <cstdio>#include <cstring>#include <algorithm>#include <cmath>#define LL long longusing namespace std;int a[42] = {0,1,1};int main(){int k;for(int i=3; i<41; i++){a[i] = a[i-1] + a[i-2];}scanf("%d", &k);int ans = 0;for(int i=1; i<=k; i++){ans += a[i];}printf("%d\n", ans);return 0;}