#include <iostream>
#include <string>

using namespace std;

int main() {

    int length;
    scanf("%d ", &length);
    string in;
    cin >> in;

    int start = 0;
    int end = length - 1;

    int osszeg = 0;
    while (start < end) {
        if (in[start] == in[end]) {
            end--;
        } else {
            osszeg++;
        }

        start++;
    }

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

    return 0;
}