“13:46 And because there are so many dimensions, one for each of these weights, that the computation just becomes prohibitively expensive. Luckily, there was an insight that lets us do this in a very reasonable time. And that’s that, if we’re careful about how we design our neural network, we can calculate the slope directly, the gradient, we can figure out the direction that we need to adjust the weight without going all the way back through our neural network and recalculating”