n = int(input())
seq = input()

m = [[{seq[0]: 1}]]
for i in range(1, n):
    m[0].append(dict(m[0][-1]))
    if seq[i] in m[0][-1]:
        m[0][-1][seq[i]] += 1
    else:
        m[0][-1][seq[i]] = 1

for i in range(1, n):
    m.append([0] * i)
    for j in range(i, n):
        m[i].append(dict(m[i - 1][j]))
        if m[i][-1][seq[i - 1]] == 1:
            del m[i][-1][seq[i - 1]]
        else:
            m[i][-1][seq[i - 1]] -= 1

for i in range(n-1, -1, -1):
    for j in range(0, n-i):
        y = i + j
        x = j
        counter = 0
        total_len = 0
        found = True
        for k, v in m[x][y].items():
            if v % 2 == 1:
                counter += 1
                if counter == 2:
                    found = False
                    break
                
            total_len += v

        if found:
            print(total_len)
            exit(0)