class Solution {
    public int fib(int N) {
        if (N == 0) {
            return 0;
        }
        if (N <= 2) {
            return 1;
        }

        int a = 1;
        int b = 1;
        int t;

        for (int i = 2; i < N; i++) {
            t = b;
            b += a;
            a = t;
        }

        return b;
    }
}