B'You are given a graph with n nodes and m directed edges. One lowercase letter is assigned to each node. We define a path 's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is 3 . Your task is find a path whose value is the largest. The first line contains two positive integers n, m ( 1 <= q n, m <= q 300 ,000 ), denoting that the graph has n nodes and m directed edges. The second line contains a string s with only lowercase English letters. The i -th character is the letter assigned to the i -th node. Then m lines follow. Each line contains two integers x, y ( 1 <= q x, y <= q n ), describing a directed edge from x to y . Note that x can be equal to y and there can be multiple edges between x and y . Also the graph can be not connected. Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead. In the first sample, the path with largest value is 1 to 3 to 4 to 5 . The value is 3 because the letter 'a ' appears 3 times. '...