A11583. Fibonacci String Subsequences

You are given a binary string $s$ (each character of this string is either 0 or 1).

Let's denote the cost of string $t$ as the number of occurences of $s$ in $t$ . For example, if $s$ is 11 and $t$ is 111011, then the cost of $t$ is $3$ .

Let's also denote the Fibonacci strings sequence as follows:

- $F(0)$ is 0;
- $F(1)$ is 1;
- $F(i)=F(i-1)+F(i-2)$ if $i>1$ , where $+$ means the concatenation of two strings.

Your task is to calculate the sum of costs of all subsequences of the string $F(x)$ . Since answer may be large, calculate it modulo $10^{9}+7$ .

The first line contains two integers $n$ and $x$ ( $1<=n<=100$ , $0<=x<=100$ ) — the length of $s$ and the index of a Fibonacci string you are interested in, respectively.

The second line contains $s$ — a string consisting of $n$ characters. Each of these characters is either 0 or 1.

Print the only integer — the sum of costs of all subsequences of the string $F(x)$ , taken modulo $10^{9}+7$ .