from keras.models import Sequential
from keras.layers import Flatten, Dense, Dropout

model = Sequential()
model.add(vgg16)
model.add(Flatten())
model.add(Dense(256, activation = 'relu'))
model.add(Dropout(0.5))
model.add(Dense(1, activation = 'sigmoid'))

vgg16.trainable = False

print(model.summary())
