class Solution

{

    //Function to count number of ways to reach the nth stair.

    int mod = 1000000007;

    int countWays(int n)

    {

        int[] dp = new int[n];

        Arrays.fill(dp, -1);

        return helper(n, 0, dp); 

    }

    int helper(int n, int current, int[] dp) {

        if (current > n)

            return 0;

        if (current == n)

            return 1;

        if (dp[current] != -1)

            return dp[current];

        return dp[current] = (helper(n, current + 1, dp) + helper(n, current + 2, dp))%mod; 

    }

}