D = input()
cal = ['x'] * D + list(raw_input()) + list(raw_input()) + ['x'] * D
ans = 0

for i in range(len(cal)-D):
    #print i
    #print cal
    calnow = cal[:i] + ['o'] * D + cal[i+D:]
    maxnow = max([len(item) for item in ''.join(calnow).split('x')])
    ans = max(ans, maxnow)

print ans