The discrete random variable R takes the values in S = { -3, -1, 1, 3 } with probabilities respectively,

( 1 - theta )/4, ( 1 - 3theta) /4, ( 1 + 3theta)/4, (1+ theta)/4,

where theta is a real constant. Find the range of values for which this is a valid probability mass function.

i think the answer is -1/3 =< theta =< 1/3

BUT HOW DO PROVE IT!!!!!!!

help!!