#include<fstream.h>
int main()
{int a, b, n, c, k;
ifstream f("nunta.in");
ofstream g("nunta.out");
f>>n;
a=1;
b=2;
k=3;
while (k<=n){c=a+b; a=b; b=c; k++;}
g<<c;
f.close();
g.close();
return 0;
}