import sys
input = lambda:sys.stdin.readline().strip()

n = int(input())
a = "0" + input()

i = j = 1
ans = 0
cur = []

while(j<=n):
    if a[j] not in cur:
        cur.append(a[j])
        ans=max(ans,j-i+1)
        j+=1
    else:
        cur.remove(a[i])
        i+=1
print(ans)