#!/usr/bin/python3

D = int(input())
C = input()
C += input()

tar = 'o'

cntarray = []

cnt = 0

for i in range(len(C)):
    if C[i] is tar:
        cnt = cnt+1
    else:
        cntarray.append(cnt)
        cnt = 1
        tar = 'x' if (tar is 'o') else 'o'


cntarray.append(cnt)

ma = 0

for i in range(0, len(cntarray)-2, 2):
    if cntarray[i+1] <= D:
        ma = max(ma, cntarray[i]+cntarray[i+1]+cntarray[i+2])

if len(cntarray) % 2 is 0 and cntarray[-1] <= D:
    ma = max(ma, cntarray[-1] + cntarray[-2])

print(ma)