D = input()
s = raw_input()+raw_input()
o = map(len, filter(None, s.split('x')))
x = map(len, filter(None, s.split('o')))
if s.startswith('x'):
    x.pop(0)
if s.endswith('x'):
    x.pop()
ans = max(o)+D
for i in xrange(len(x)):
    if x[i] <= D:
        ans = max(ans, o[i]+x[i]+o[i+1])
print ans