Pasha pirdr

fin  = open("marks.in")
fout = open("marks.out","w")

N = int(fin.readline())
DP5 = 1
DP2 = 1
for i in range(1,N):
    A = DP5
    B = DP2
    DP5 = A + B
    DP2 = A

fout.write(str(DP5+DP2))

fin.close()
fout.close()