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Write a Python function to calculate the Negative Predictive Value (NPV) of Red Hen classification guesses / predictions for audio clips.

Please see: https://en.wikipedia.org/wiki/Confusion_matrix

Assume the existence of the variables p, n, tp, tn, fp, and fn.

For example, a function that calculates the True Positive Rate (TPR) is:

def tpr(tp, fn):
    return tp / (tp + fn)