The 15th youth informatics competition in Shushan District in 2019

The first question is Sum up

【 Problem description 】

Fiboracci series , Also called golden section series . Because mathematician Leonardo · Fibonacci introduced, for example, rabbit breeding , It is also called “ Rabbit Series ”, It refers to such a sequence ：1、1、2、3、5、8、13、21、34、······ In Mathematics , Fibonacci sequence is defined recursively as follows ：F(1)=1,F(2)=1,F(n)=F(n-1)+F(n-2)(n>=3).

The first two items of Fibonacci series are 1,1. Each subsequent item is the sum of the first two items . Now the input n, Before Fibonacci sequence n Sum of items (1<=n<=5000).

【 Input format 】sum.in

Enter only one number n, It means finding the front of Fibonacci sequence n Sum of items .

【 Output format 】sum.out

Before outputting Fibonacci series n Sum of items

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