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#include <iostream>
#include <vector> 
#include <algorithm>

#define ll long long
#define push push_back

using namespace std;

int main() {
    string p;
    cin >> p;

    int N = p.size();

    int i = 0;
    int j = N-1;

    int res = 0;

    int swa = 0;
    int swat = 0;

    while(i < j) {
        if(swa > 0) {
            swa--;
            res++;
        }else if(p[i] != p[j]) {
            swa++;
            swat++;
        }

        j--;
        i++;
    }

    if(i == j && swa > 0) {
        swa--;
        res++;
    }

    cerr << "swa: " << swa << " | swat: " << swat <<  " | res: " << res << "\n"; 

    if(swat >= 1 && res == 0) {
        res = -1;
    }

    cout << res << "\n";

    // abbaaab
    // abb a aab
    // ab
    //
    // res = 0
    // swa = 1
}