#include <fstream.h>

int min(int a, int b)
{
	if (a < b)
		return a;
	else
		return b;
}

int main()
{
	ifstream input;
	ofstream output;
	int n, i, length;
	int additions1[5100];
	int additions2[5100];
	int additions3[5100];
	input.open ("palin.in");
	output.open ("palin.out");
	char string[5100];

	input >> n;
	input >> string;

	for (i = 0; i < n; i++)
	{
		additions1[i] = 0;
		if (i != n-1)
		{
			if (string[i] == string[i+1])
				additions2[i] = 0;
			else
				additions2[i] = 1;
		}
	}

	for (length = 3; length <= n; length++)
	{
		for (i = 0; i <= n-length; i++)
		{
			if (string[i] == string[i+length-1])
			{
				additions3[i] = 1 + min(additions2[i],
						    additions2[i+1]);
				additions3[i] = min(additions3[i],
						    additions1[i+1]);
			}
			else
				additions3[i] = 1 + min(additions2[i],
							additions2[i+1]);
		}

		for (i = 0; i < n; i++)
		{
			additions1[i] = additions2[i];
			additions2[i] = additions3[i];
		}
	}

	output << additions3[0] << "\n";

	input.close();
	output.close();
	return 0;
}