import std.stdio;
import std.conv;
import std.string;
import std.algorithm;

void main(){
  auto n = readln.strip.to!int;
  auto memo = new long[](n+1);
  memo[1..3] = [1, 2];
  foreach(i; 3..n+1){
    memo[i] = memo[i-1] + memo[i-2];
  }
  memo[n].writeln;
}