ACMSGURU 123 - The sum

The sum

Problem Description

The Fibonacci sequence of numbers is known: F1 = 1; F2 = 1; Fn 1 = Fn Fn-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

Solution

代码语言：javascript复制

#include <bits/stdc  .h>

int main() {
    std::ios::sync_with_stdio(false);

    int k;
    std::cin >> k;

    unsigned long long a = 1;
    unsigned long long b = 1;

    unsigned long long res = 0;
    for(int i = 1; i <= k; i  ) {
        if(!(i == 1 || i == 2)) {
            auto tmp = a;
            a  = b;
            b = tmp;
        }
        res  = a;
    }

    std::cout << res << std::endl;

    return 0;
}

fibonacci input line numbers sample

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