# Equidistribution of returns and height of first peak of Dyck paths

I believe that it is "well known" that the following two statistics on Dyck paths have symmetric joint distribution:

1. number of returns to the axis $RET(D)$
2. height of the first peak (or length of the last descent) $HFP(D)$

That is: $\sum_{D} x^{RET(D)}y^{HFP(D)} = \sum_{D} x^{HFP(D)}y^{RET(D)}$

However, I could not find a reference for that. Might it be due to Kreweras?

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