proof for double summation formula

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I am looking for a proof for this formula:

$$\sum_{t=1}^N\sum_{s=1}^N f(t-s) = \sum_{k=-N+1}^{N-1} (N-|k|)f(k)$$

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... and from $k = 1-N$ to $N-1$.

Hint: how many pairs $(s,t)$ are there with $t-s = k$?