I bought this bike from a guy about a month or two ago and my chain fell off the sprocket so i put it back on and it kept on falling off, so then i turned my bike over and notice that my bikes sprocket is bent and got a hammer and made it straight, i went up and down my street two times and on the third time it fell off, i don't have the money to buy a new sprocket. So how can i deal with this? I need to ride my bike to stay fit.