from re import findall

D = input()
C = raw_input() + raw_input()
C = [0, 14] + map(len, findall('o+|x+', C)) + [14, 0]
ans = 0
for i in range(1, len(C)-1, 2):
    if C[i] <= D:
        ans = max(ans, C[i-1] + C[i] + C[i+1])
    elif C[i-1] < C[i+1]:
        ans = max(ans, D + C[i+1])
    else:
        ans = max(ans, D + C[i-1])
print(ans)