import std.stdio, std.string, std.conv;
import std.functional;

int n;
ulong ans;


alias foo = memoize!hoge;

void main(){
    n = readln.strip.to!int;
    
    foo(n+1).writeln;
    
}

ulong hoge(int num){
    if(num == 1) return 1;
    if(num < 0) return 0;
    return foo(num-1) + foo(num-2);
}