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#include "bits/stdc++.h" // Tomasz Nowak
using namespace std;     // University of Warsaw
using LL = long long;
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
#define ssize(x) int(x.size())
template<class A, class B> auto& operator<<(ostream &o, pair<A, B> p) {
	return o << '(' << p.first << ", " << p.second << ')';
}
template<class T> auto operator<<(ostream &o, T x) -> decltype(x.end(), o) {
	o << '{'; int i = 0; for(auto e : x) o << (", ")+2*!i++ << e; return o << '}';
}
#ifdef DEBUG
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n'
#else
#define debug(...) {}
#endif

int main() {
	cin.tie(0)->sync_with_stdio(0);

	string s;
	cin >> s;
	map<char, int> cnt;
	for(char c : s)
		++cnt[c];
	if(int(cnt['a'] % 2) + int(cnt['b'] % 2) != int(ssize(s) % 2)) {
		cout << "-1\n";
		return 0;
	}

	deque<pair<char, int>> compressed = {{s[0], 1}};
	FOR(i, 1, ssize(s) - 1)
		if(s[i] == compressed.back().first)
			++compressed.back().second;
		else
			compressed.emplace_back(s[i], 1);

	LL ans = 0;
	while(ssize(compressed) >= 2) {
		char end = compressed.back().first;
		if(compressed[0].first == end) {
			--compressed[0].second;
			if(compressed[0].second == 0)
				compressed.pop_front();
		}
		else if(ssize(compressed) == 2 and compressed[1].second == 1)
			ans += compressed[0].second / 2;
		else {
			ans += compressed[0].second;
			--compressed[1].second;
			if(compressed[1].second == 0) {
				auto fr = compressed.front();
				compressed.pop_front();
				compressed.pop_front();
				if(not compressed.empty()) {
					assert(compressed.front().first == fr.first);
					compressed.front().second += fr.second;
				}
				else
					compressed.emplace_front(fr);
			}
		}
		--compressed.back().second;
		if(compressed.back().second == 0)
			compressed.pop_back();
	}

	cout << ans << '\n';
}