n = int(input())

a=[]
for i in range(n):
    if i==0:
        a.append(2)
    elif i==1:
        a.append(3)
    else:
        a.append(a[i-2] + a[i-1])

print(a[n-1])
# n = int(input())
# a = [0] * n  

# for i in range(n):
#     if i == 0:
#         a[0] = 2
#     elif i == 1:
#         a[1] = 3
#     else:
#         a[i] = a[i - 2] + a[i - 1]

# print(a[n-1])

两个，一个是不先设置list维度直接用append，一个是设置空列表[0]*n