This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <iostream>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;
const int maxn = 1e6 + 5, abc = 26;
int ans = 0;
int pf[abc][abc], pfmin[abc][abc], last[abc][abc];
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n;
	cin >> n;
	string s;
	cin >> s;
	for (int i = 0; i < abc; i++) for (int j = 0; j < abc; j++) pfmin[i][j] = 1e9;
	for (int i = 0; i < n; i++)
	{
		int a = s[i] - 'a';
		for (int b = 0; b < abc; b++) if (a ^ b)
		{
			pf[a][b]++;
			ans = max(ans, pf[a][b] - pfmin[a][b]);
			pfmin[b][a] = min(pfmin[b][a], last[b][a]);
			last[b][a] = --pf[b][a];
			ans = max(ans, pf[b][a] - pfmin[b][a]);
		}
	}
	cout << ans << "\n";
	return 0;
}
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