The French prefix a- is added to adjectives and nouns to create their antonyms; it's sometimes called the "alpha privative" (a privatif). The most common English equivalents are a- and non-.
There's a bit of debate with regard to accents on capital letters. Most everyone agrees that accents are required on lower case letters, but some French speakers claim that they are unneccessary or even incorrect on capital letters. Is this true? The answer depends in part on who you ask.