WASHINGTON (AP) -- President Barack Obama says being a dad is sometimes his hardest job, but also the most rewarding.Just ahead of Father's Day, the president devoted his Saturday radio and Internet address to fatherhood. He talked about growing up without a dad, his own failings as a father and the values he hopes to teach his daughters Malia, 12, and Sasha, 10.He described the responsibilities that all fathers have to their children and said his administration is trying to help during tough economic times and long deployments for U.S.