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#pragma GCC optimize ("Ofast")
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define FOR(i, a, b) for (auto i=(a); i<(b); i++)
#define FORD(i, a, b) for (int i=(a); i>(b); i--)
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define PPC(x) __builtin_popcountll(x)
#define MSB(x) (63 - __builtin_clzll(x))
#define LSB(x) __builtin_ctzll(x)
#define ARG(x, i) (get<i>(x))
#define ithBit(m, i) ((m) >> (i) & 1)
#define pb push_back
#define ft first
#define sd second
#define kw(a) ((a) * (a))
#ifdef DEBUG
#include "debug.h"
#else
#define dbg(...) 0
#endif
using namespace std; 

template <typename T1, typename T2> inline void remin(T1& a, T2 b) { a = min(a, (T1)b);	}
template <typename T1, typename T2> inline void remax(T1& a, T2 b) { a = max(a, (T1)b);	}

const int maxN = 1 << 18;

char T[maxN];

void solve()
{
	scanf ("%s", T);
	int n = strlen(T);
	vector <int> A, B;
	FOR(i, 0, n)
		(T[i] == 'a' ? A : B).pb(i);
	
	vector <int>& x = A.size() % 2 == 1 ? B : A;
	long long res = 0;
	
	FOR(i, 0, SZ(x) / 2)
	{
		int p = x[i], q = x[SZ(x) - 1 - i];
		p = p + 1;
		q = n - q;
		res += abs(p - q);
	}
	
	if (SZ(x) % 2 == 1)
		res = -1ll;
		
	printf("%lld\n", res);
}
 
int main()
{
	int t = 1;
	//scanf ("%d", &t);
	FOR(tid, 1, t+1)
	{
		//printf("Case #%d: ", tid);
		solve();
	}
	return 0;
}