n, k = list(map(int, input().split(' ')))
s = input()
s_i = [i for i in range(1, n + 1)]

stack = queue.LifoQueue()
stack_i = queue.LifoQueue()

stack.put(s[0])
stack_i.put(s_i[0])

i = 1
result = -1
while(i < n):
    val = s[i]
    val_i = s_i[i]
    top = stack.get()

    if top != val:
        temp = stack_i.get()
        if temp == k:
            result = val_i
            break
        elif val_i == k:
            result = temp
            break
    else:
        stack.put(top)
        stack.put(val)
        stack_i.put(val_i)
    i += 1

print(result)