#include <stdio.h>
#include <string.h>
int main() {
    int a[5000], b[5000], c[5000];
    int n;
    int num = 1;
    scanf("%d", &n);
    if (n < 3) {
        printf("%d\n", n);
        return 0;
    }
    a[1] = 1; b[1] = 2;//初始化数组a,数组b 
    for (int i = 3; i <= n; i++) {
        for (int j = 1; j <= num; j++) {
            c[j] = a[j] + b[j];
        }
        for (int j = 1; j <= num; j++) {
            if (c[j] > 9) { // 进位
                c[j + 1] += c[j] / 10;
                c[j] %= 10;
                if (j + 1 > num) {
                    num++;
                }
            }
        }
        for (int j = 1; j <= num; j++) {
            a[j] = b[j];
        }
        for (int j = 1; j <= num; j++) {
            b[j] = c[j];
        }
    }
    for (int i = num; i > 0; i--) {
        printf("%d", c[i]);
    }
    return 0;
}