# coding:utf-8

D = int(input())
C = 'xxxxxxxxxxxxxxx' + input()
C = C + input() + 'xxxxxxxxxxxxxxxxx'
ans = [0]
for i in range(len(C)+1):
    for j in range(i, len(C)+1):
        e = C[i:j]
        # xの連続チェック
        flag = 0
        for f in range(1, len(e)):
            if e[f-1] == 'x' and e[f] == 'o':
                flag = 1
            if e[f-1] == 'o' and e[f] == 'x' and flag == 1:
                flag = 2
                break
        # xの数
        if flag != 2:
            if D > 0:
                if 1 <= e.count('x') <= D:
                    ans.append(len(e))
            if D == 0:
                if e.count('x') == 0:
                    ans.append(len(e))

print(max(ans))