Staircase

Chef thinks he has put on some weight from eating too much. But it's too difficult for him to be on a diet. So, he decides to take the stairs every time instead of elevators. You’re given a positive integer $N$, which is the number of steps in the staircase. Each time Chef can climb either $a$ or $b$ steps. Chef wonders how many unique ways are there for him to climb the staircase. And so, he