def f(num):
    if num<1:
        return -1
    elif num==1 or num==2 :
        return 1
    else :
        return f(num-1)+f(num-2)
a=int(input())
r=f(a)
if a!= -1:
    print(r)

/**************************************************************
	Problem: 1238
	User: admin
	Language: Python
	Result: Accepted
	Time:350 ms
	Memory:34480 kb
****************************************************************/