SGU123 The sum

123. The sum

time limit per test: 0.5 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

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Author	: Paul "Stingray" Komkoff, Victor G. Samoilov
Resource	: 5th Southern Subregional Contest. Saratov 2002
Date	: 2002-10-10

题解：迭代一下就行。。。数组都不用开。。。

#include<stdio.h>
int main(void)
{
    long f1,f2,f3,k,i;
    long long ans;
    scanf("%ld",&k);
    if(k==1)
    {
        printf("1\n");
        return 0;
    }
    ans=2;
    i=3;
    f1=1;
    f2=1;
    while(i<=k)
    {
        f3=f1+f2;
        ans+=f3;
        f1=f2;
        f2=f3;
        i++;

    }
    printf("%lld\n",ans);
    return 0;
}