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// Karol Kosinski 2022
#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i=(a),_b=(b);i<_b;++i)
#define FR_(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FD_(i,b,a) for(int i=(b),_a=(a);i>=_a;--i)
#define ALL(c) (c).begin(),(c).end()
#define SIZE(c) int((c).size())
#define TIE(x...) int x;tie(x)
#define X first
#define Y second
#ifndef ENABLE_DEBUG
#define DEB(k,p,f,x...)
#else
#define DEB(k,p,f,x...) {if(k)printf("--------%4d : %s\n",__LINE__,__FUNCTION__);if(p)f(x);}
#endif
#define DEBL DEB(1,1,void,0)
#define DEBF(f,x...) DEB(1,1,f,x)
#define DEBC(p,x...) DEB(0,p,printf,x)
#define DEBUG(x...) DEB(0,1,printf,x)
using namespace std;
using LL = long long;
using ULL = unsigned long long;
using PII = pair<int, int>;
using TIII = tuple<int, int, int>;

constexpr int NX = 200'005;

char A[NX], B[NX];

LL solve(int n, int k, char c)
{
    LL res = 0;
    copy( A, A + n, B );
    reverse( B, B + n );
    int i = 0, j = 0;
    while ( k -- )
    {
        while ( A[i] != c ) ++ i;
        while ( B[j] != c ) ++ j;
        res += abs( i - j );
        ++ i, ++ j;
    }
    return res;
}

int main()
{
    scanf("%s", A);
    int n = strlen(A);
    int an = count( A, A + n, 'a' ), bn = count( A, A + n, 'b' );
    if ( an % 2 == 0 )
        printf("%lld\n", solve( n, an / 2, 'a' ) );
    else if ( bn % 2 == 0 )
        printf("%lld\n", solve( n, bn / 2, 'b' ) );
    else
        printf("-1\n");
    return 0;
}