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#include <bits/stdc++.h>

#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define st first
#define nd second

using namespace std;
 
#define sim template < class c
#define ris return * this
#define dor > debug & operator <<
#define eni(x) sim > typename \
	enable_if<sizeof dud<c>(0) x 1, debug&>::type operator<<(c i) {
sim > struct rge { c b, e; };
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c* x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug {
#ifdef LOCAL
~debug() { cerr << endl; }
eni(!=) cerr << boolalpha << i; ris; }
eni(==) ris << range(begin(i), end(i)); }
sim, class b dor(pair < b, c > d) {
	ris << "(" << d.first << ", " << d.second << ")";
}
sim dor(rge<c> d) {
	*this << "[";
	for (auto it = d.b; it != d.e; ++it)
	*this << ", " + 2 * (it == d.b) << *it;
	ris << "]";
}
#else
sim dor(const c&) { ris; }
#endif
};
#define imie(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
 
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vll = vector<ll>;

ll Solve() {
	string s;
	cin >> s;
	int n = SZ(s);
	
	if (count(ALL(s), 'a') % 2 == 1 && count(ALL(s), 'b') % 2 == 1) {
		return -1;
	}
	
	list<pair<char, int>> L;
	char last_ch = s[0];
	int cnt_ch = 1;
	for (int i = 1; i < n; ++i) {
		if (s[i] == last_ch) {
			++cnt_ch;
		} else {
			L.emplace_back(last_ch, cnt_ch);
			last_ch = s[i];
			cnt_ch = 1;
		}
	}
	L.emplace_back(last_ch, cnt_ch);
	
	ll answer = 0;
	
	auto DropChar = [&](list<pair<char, int>>::iterator it) {
		if (--it->second == 0) {
			if (it != L.begin() && next(it) != L.end()) {
				const int merge_left = prev(it)->second;
				L.erase(prev(it));
				next(it)->second += merge_left;
			}
			L.erase(it);
		}
	};
	
	while (SZ(L) > 1) {
		if (L.front().st == L.back().st) {
			DropChar(L.begin());
			DropChar(prev(L.end()));
		} else if (L.front().nd < L.back().nd) {
			answer += L.front().nd;
			DropChar(prev(L.end()));
			DropChar(next(L.begin()));
		} else {
			answer += L.back().nd;
			DropChar(L.begin());
			DropChar(prev(prev(L.end())));
		}
	}
	
	return answer;
}

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout << fixed << setprecision(11);
	cerr << fixed << setprecision(6);
	
	cout << Solve() << "\n";
}