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Minutes later, Erding almost added a second but he was denied by Ochoa who made a good save to stop the striker's header after he latched onto Benjamin Corgnet's cross into the box.
However, Erding was not to be denied and he got his side's second just before the break with Gradel again the provider and the striker made no mistake slotting the ball home.
After the break, Erding could have added a third for his side as he lauched a shot from outside the box which was saved again by Ochoa.
The home side continued to press and eventually they got their third goal when Perrin managed to turn the ball from inside the box after Ajaccio had failed to clear their lines from a corner.
Saint-Etienne held out for the three points and secured their place in the Europa League for next season.