import sys

k=int(input())

# s=list(range(1,11))
# s[0],s[1]=2,3

# for i in range(0,10):
#     if i>1:
#         s[i]=s[i-1]+s[i-2]

# print(s[k-1])

def kk(s):
    if s==1:
        return 2
    elif s==2:
        return 3
    else:
        return kk(s-1)+kk(s-2)
print(kk(k))