N = 2
A = [2,1]
A.insert(0,0)

X = 0
Y = 0
MaxT = [0, A[1]]
MaxD = [0, 0, A[1]]

if N == 1:
    print(A[1])
    exit()
else:
    MaxT.append(A[2])
    for i in range (3, N+1):
        if MaxT[i-2] >= MaxD[i-2]:
            X = MaxT[i-2] + A[i]
            Y = MaxD[i-2] + A[i-1]
            MaxT.append(X)
            MaxD.append(Y)
        else:
            X = MaxD[i-2] + A[i]
            Y = MaxD[i-2] + A[i-1]
            MaxT.append(X)
            MaxD.append(Y)

if MaxT[N] >= MaxD[N]:
    print(MaxT[N])
else:
    print(MaxD[N])