def fib(N):
    i2_i = [0, 1, 2]
    for n in range(N-2):
        i2_i[0] = i2_i[1]
        i2_i[1] = i2_i[2]
        i2_i[2] = i2_i[0] + i2_i[1]
    return i2_i[2]
        
N = input()
print fib(N)