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// Arti1990, II UWr

#include <bits/stdc++.h>

#define forr(i, n)                  for(int i=0; i<n; i++)
#define FOREACH(iter, coll)         for(auto iter = coll.begin(); iter != coll.end(); ++iter)
#define FOREACHR(iter, coll)        for(auto iter = coll.rbegin(); iter != coll.rend(); ++iter)
#define lbound(P,K,FUN)             ({auto SSS=P, PPP = P-1, KKK=(K)+1; while(PPP+1!=KKK) {SSS = (PPP+(KKK-PPP)/2); if(FUN(SSS)) KKK = SSS; else PPP = SSS;} PPP;})
#define testy()                     int _tests; scanf("%d", &_tests); FOR(_test, 1, _tests)
#define CLEAR(tab)                  memset(tab, 0, sizeof(tab))
#define CONTAIN(el, coll)           (coll.find(el) != coll.end())
#define FOR(i, a, b)                for(int i=a; i<=b; i++)
#define FORD(i, a, b)               for(int i=a; i>=b; i--)
#define MP                          make_pair
#define PB                          push_back
#define ff                          first
#define ss                          second
#define deb(X)                      X;

#define M 1000000007
#define INF 1000000007LL

using namespace std;

int n, ilea, ileb;
char slowo[1000007];

long long solve()
{
    scanf("%s", slowo);
    n = strlen(slowo);
    forr(i, n)
    {
        ilea += slowo[i] == 'a';
        ileb += slowo[i] == 'b';
    }
    char c = (ilea%2 == 0 ? 'a' : (ileb%2 == 0 ? 'b' : -1));
    if(c == -1)
        return -1;
    if(ilea == 0 || ileb == 0)
        return 0;

    long long res = 0;
    int p = 0, k = n-1;
    while(true)
    {
        while(slowo[p] != c)
            p++;
        while(slowo[k] != c)
            k--;
        if(k <= p)
            return res;
        res += abs(n-1-(p+k));
        p++;
        k--;
    }

    return 0;
}

int main()
{
    printf("%lld\n", solve());

    return 0;
}