Calculate the concentration (mol dm^-3) of an aqueous sol'n of H2SO4 with a pH of 0.25?

Ok I got the answer of 0.281 mol dm^-3 using Ph=-log(hydrogen concentration) and getting a H+ value of 0.562341 and then halving it :/ Apparently it's wrong and I don't know how to do it, so what was I meant to do? thanks.