#include <set>

#include <cstdio>

#include <algorithm>

#define min(x, y) ((x) < (y) ? (x) : (y))

#define max(x, y) ((x) > (y) ? (x) : (y))

#define N 1100000

int n, pos, maxright, ans;

int p[N], f[N];

char s[N];

std::set <int> S;

std::set <int> :: iterator it;

struct node

{

	int id, p;

}a[N];

inline bool cmp(node x, node y)

{

	return x.p < y.p;

}

inline void manacher()

{

	int i;

	s[0] = '$', s[n * 2 + 1] = '#';

	for(i = n; i >= 1; i--) s[i * 2] = s[i], s[i * 2 - 1] = '#';

	for(i = 1; i <= n * 2 + 1; i++)

	{

		p[i] = 1;

		if(i <= maxright)

			p[i] = min(p[pos * 2 - i], maxright - i + 1);

		while(s[i - p[i]] == s[i + p[i]]) p[i]++;

		if(maxright < i + p[i] - 1)

		{

			pos = i;

			maxright = i + p[i] - 1;

		}

	}

	for(i = 1; i <= n; i++) f[i] = (p[i * 2 - 1] - 1) / 2;

}

int main()

{

	int i, j;

	scanf("%d", &n);

	scanf("%s", s + 1);

	manacher();

	for(i = 1; i <= n; i++) a[i].id = i, a[i].p = i - f[i];

	std::sort(a + 1, a + n + 1, cmp);

	j = 1;

	for(i = 1; i <= n; i++)

	{

		while(a[j].p <= i && j <= n)

			S.insert(a[j].id), j++;

		it = S.upper_bound(i + f[i] / 2);

		if(it == S.begin()) continue;

		it--;

		ans = max(ans, (*it - i) * 4);

	}

	printf("%d\n", ans);

	return 0;

}