import kotlin.math.max

fun main() {
    val builder = StringBuilder()

    val d = readInputLine().toInt()

    val c = readInputLine() + readInputLine()

    val runLength = runLength(c).toMutableList()

    if (runLength[0].first == 'x') {
        runLength[0] = runLength[0].copy(second = 15)
    } else {
        runLength.add(0, Pair('x', 15))
    }

    if (runLength[runLength.lastIndex].first == 'x') {
        runLength[runLength.lastIndex] = runLength[runLength.lastIndex].copy(second = 15)
    } else {
        runLength.add(Pair('x', 15))
    }

    var ans = 0

    val arr = runLength.toTypedArray()

    for ((i, p) in arr.withIndex()) {
        if (p.first != 'o') {
            continue
        }

        if (arr[i - 1].second >= d) {
            if (arr[i - 1].second == d) {
                ans = max(ans, p.second + d + arr[i - 2].second)
            } else {
                ans = max(ans, p.second + d)
            }
        }

        if (arr[i + 1].second >= d) {
            if (arr[i + 1].second == d) {
                ans = max(ans, p.second + d + arr[i + 2].second)
            } else {
                ans = max(ans, p.second + d)
            }
        }
    }

    builder.appendln(ans)

    print(builder.toString())
}

fun readInputLine(): String {
    return readLine()!!
}

fun runLength(s: String): List<Pair<Char, Int>> {
    val ret = mutableListOf<Pair<Char, Int>>()

    if (s.isNotEmpty()) {
        var prev = s[0]
        var cnt = 0

        for (c in s) {
            if (c == prev) {
                cnt++
            } else {
                ret.add(Pair(prev, cnt))
                prev = c
                cnt = 1
            }
        }

        ret.add(Pair(prev, cnt))
    }

    return ret.toList()
}