def weight(c):
    return ord(c)-ord('a')+1

def main():
    n, k, = [int(i) for i in input().split()]
    string = [i for i in input()]
    string.sort(key=weight)
    
    total_weight = 0
    last_weight = -1
    curr = 0
    for i in string:
        if curr == k:
            break
        if weight(i) > last_weight+1:
            total_weight += weight(i)
            last_weight = weight(i)
            curr += 1
    if curr == k:
        print(total_weight)
    else:
        print(-1)


main()