Check Engine Light...Please help

1997 Buick Century.
My check engine light came on and the code is P0140 which is B1 S2 faulty O2 sensor. I went to buy a new one but both places I went to asked me if the O2 sensor was located before or after the catalytic converter. How do I determine this? Someone please help me.

My Haynes manual states that: "All models are equipped with two oxygen sensores; one upstream oxygen sensor and one downstream oxygen sensor" The upstream oxygen sensor(mounted in the exhaust system before the catalytic converter). The downstream oxygen sensor (mounted in the exhaust system after the catalytic converter)
Hope this help, good luck.

Bank one sensor 2 would be the odd bank of cylinders post convertor. Sensor one is always the senor at the exhaust manifold, sensor 2 is always after the cat convertor.

Ok, so I need the O2 sensor that is after the catalytic converter.
Thanks a lot Elzie and Flatrater.