#include <stdio.h>

unsigned long long int m[50];

int main()
{

        int N;
        scanf( "%d\n", &N );
        m[1] = 1;
        m[2] = 2;
        for( int i = 3; i <= N; i++ ) {
                m[i] = m[i-1] + m[i-2];
        }
        printf("%lld\n", m[N] );

        return 0;
}