static long dp[] = new long[37];
    static long M = 10000000007L;
    public static long stepPerms(int n) {
        if(n<3)return n;
        if(n==3)return 4;
        if(dp[n]>0)return dp[n];
        long ans=(stepPerms(n-1)+stepPerms(n-2)+stepPerms(n-3))%M;
        dp[n] = ans;
        return ans;
    }