C :

#include<stdio.h>
int main(void)
{
	int x1,x2,x3,y1,y2,y3,N;
	scanf("%d",&N);
	if(N==1)
		printf("2\n");
	else
	{
		if(N==2)
			printf("3.5\n");
		else
		{
			int i;
			double s=3.5;
			x1=2; x2=3;
			y1=1; y2=2;
			for(i=3;i<=N;i++)
			{
				x3=x1+x2; y3=y1+y2;
				s+=x3*1.0/y3;
				x1=x2;  x2=x3;
				y1=y2;  y2=y3;
			}
			printf("%.2f\n",s);
		}
	}
}

C++ :

#include <bits/stdc++.h>
using namespace std;
long long n;
float s,a=2,b=1,t;
int main()
{
    cin>>n;
    for(int i=1;i<=n;i++)
    s=s+1/b*a,t=b+a,b=a,a=t;
    printf("%.2f",s);
    return 0;
}