题解 | #小乐乐走台阶#

n=int(input())

def fn(n):
    f=[0 for i in range(n)]
    for i in range(n):
        if i==0:
            f[0]=1
        elif i==1:
            f[1]=2
        else:
            f[i]=f[i-1]+f[i-2]
    return f
f=fn(n)
print(f[n-1])