A CRJ200 MISSED A XING RESTR WHEN THE FLT CREW IMPROPERLY PROGRAMMED THE FMC.

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ON DSCNT INTO JFK; WE WERE ISSUED A XING RESTR 15 DME E OF ROBER. THE CAPT PUT THE XING INTO THE FMS AND THE ALT WAS PUT INTO THE ALT SELECTOR. WE WERE TOLD TO CROSS AT 250 KTS AND 12000 FT. WE VERBALLY CONFIRMED THE XING. I SAW THE CAPT PUT IT INTO THE FMS. HOWEVER; AT 15 DME E OF ROBER; ATC QUERIED US AND ASKED IF WE WERE GOING TO MAKE THE RESTR. WE QUICKLY REALIZED THE XING WAS PUT INCORRECTLY INTO THE FMS. INSTEAD OF PUTTING THE '-' INTO THE FMS; THE CAPT HAD HIT THE BUTTON TWICE WHICH MADE IT '+' 15 DME WHICH IS NOW W OF ROBER. WE TOLD ATC WE WOULD DO OUR BEST INTO THE DSCNT AND HE SWITCHED US TO NY APCH. THE REST OF THE FLT WAS UNEVENTFUL. I AM NOT SURE HOW WE BOTH MISSED THE XING. I KNOW I SHOULD HAVE VERIFIED IT MORE CLOSELY AND FOCUSED MORE ON THE XING INSTEAD OF RELYING ON THE FMS. HOWEVER; A CONTRIBUTING FACTOR IN MY OPINION WAS THE FACT THAT THE OVERNIGHT WAS REDUCED REST TO 8 HRS IN BOS. THE CAPT HAD A 14 HR DUTY DAY THE DAY BEFORE AND I WAS ON DAY 5 OF MY RESERVE BLOCK AND WE BOTH FELT TIRED THAT MORNING. I FEEL THAT IF I HAD BEEN MORE FRESH I PROBABLY WOULD HAVE CAUGHT OUR MISTAKE.