D = int(input())

X = [0 if i == 'x' else 1 for i in input()] + [0 if i == 'x' else 1 for i in input()] + [0]

ans = 0

for i in range(0, 14- D):
    
    x = X.copy()
    for j in range(i, i + D):
        x[j] = 1
        
    a = 0
    b = 0
    for j in x:
        if j == 1:
            b += 1
        else:
            a = max(a, b)
            b = 0
            
    ans = max(ans, a)
    
print(ans)