#include<iostream>
using namespace std;

int main(){   
    long long n,a=1,b=1,c;
    cin>>n;//千万OI一场空，不开longlong见祖宗 
    for(int i=3;i<=n;i++){
        c=a+b;a=b;b=c;
    }printf("%.2lf",c);
    return 0;
}