Timed Exam

Leo plans to take a timed exam. He fills the corresponding bubble in the answer sheet right after completing each problem. If he spends 2 minutes to complete each problem, he would have 6 problems on the answer sheet left empty. If he spends 1 minute on each problem, he would have 4 minutes left before the end of the exam. Let N be the number of problems in the exam. What's the least value for N?