#include <iostream>
#include <vector>
#include <cstdio>
#include <cstdlib>
#include <climits>
#include <cassert>
#include <functional>

typedef unsigned int index;

template<class Element> class RangeTree/*{{{*/
{
	private:
		Element *elements;/*{{{*/
		std::pair<index, index> *childrens;
		size_t my_size;
		std::function<Element (const Element&, const Element&)> func;/*}}}*/

		index build(index tl, size_t tr, index &last)/*{{{*/
		{
			if (tr - tl == 1)
			{
				return my_size + tl - 1;
			}
			index v = last;
			last++;
			size_t tm = (tl + tr) / 2;
			childrens[v] = {build(tl, tm, last), build(tm, tr, last)};
			elements[v] = func(elements[childrens[v].first], elements[childrens[v].second]); 
			return v;
		}/*}}}*/

		Element get(index v, index tl, size_t tr, index l, size_t r)/*{{{*/
		{
			if (l <= tl and tr <= r)
			{
				return elements[v];
			}
			size_t tm = (tl + tr) / 2;
			if (tm <= l)
			{
				return get(childrens[v].second, (index)tm, tr, l, r);
			}
			else if (tm >= r)
			{
				return get(childrens[v].first, tl, tm, l, r);
			}
			else
			{
				return func(get(childrens[v].first, tl, tm, l, r), get(childrens[v].second, (index)tm, tr, l, r));
			}
		}/*}}}*/

		void set(index v, index tl, size_t tr, index pos, const Element &el)/*{{{*/
		{
			if (pos < tl or pos >= tr)
				return;
			if (tr - tl == 1)
			{
				elements[v] = el;
				return;
			}
			size_t tm = (tl + tr) / 2;
			set(childrens[v].first, tl, tm, pos, el);
			set(childrens[v].second, (index)tm, tr, pos, el);
			elements[v] = func(elements[childrens[v].first], elements[childrens[v].second]); 
		}/*}}}*/

	public:
		template<class T> RangeTree(const T &begin, const T &end, const std::function<Element (const Element&, const Element&)> &new_func): my_size(0), func(new_func)/*{{{*/
		{
			for (T i = begin; i != end; i++)
				my_size++;
			assert(my_size > 0);
			elements = new Element[my_size * 2 - 1];
			childrens = new std::pair<index, index>[my_size - 1];
			index j = 0;
			for (T i = begin; i != end; i++, j++)
			{
				elements[j + my_size - 1] = *i;
			}
			index last = 0;
			build(0, my_size, last);
		}/*}}}*/

		Element get(index l, size_t r)/*{{{*/
		{
			return get(0, 0, my_size, l, r);
		}/*}}}*/

		size_t size()/*{{{*/
		{
			return my_size;
		}/*}}}*/
		
		void set(index pos, const Element &el)/*{{{*/
		{
			set(0, 0, my_size, pos, el);
		}/*}}}*/

};/*}}}*/

template<class Element> class PrefixSum/*{{{*/
{
private:
	Element *all;
	size_t my_size;
	std::function<Element (Element, Element)> &func, &obr_func;
public:
	template<class Iterator>/*{{{*/
	inline PrefixSum(Iterator begin, Iterator end,
			std::function<Element (Element, Element)> new_func,
			std::function<Element (Element, Element)> new_obr_func): my_size(0), func(new_func), obr_func(new_obr_func)
	{
		my_size = end - begin;
		all = new Element[my_size];
		for (index i = 0; i < my_size; i++)
			all[i] = *begin, begin++;
		for (index i = 1; i < my_size; i++)
			all[i] = func(all[i - 1], all[i]);
	}/*}}}*/

	inline Element get(index l, size_t r) const/*{{{*/
	{
		if (l == 0)
			return all[r - 1];
		else
			return obr_func(all[r - 1], all[l - 1]);
	}/*}}}*/

	inline size_t size() const/*{{{*/
	{
		return my_size;
	}/*}}}*/
};/*}}}*/

template<class Element> class SparsTable/*{{{*/
{
	private:
		size_t my_size, *my_log;
		Element **table;
		std::function<Element (Element, Element)> func;
	public:
		template <class Iterator> SparsTable(Iterator begin, Iterator end, std::function<Element (Element, Element)> new_func): func(new_func)/*{{{*/
		{
			Iterator copy = begin;
			my_size = 0;
			while (copy != end)
			{
				copy++;
				my_size++;
			}
			my_log = new size_t[my_size + 1];
			size_t cur = 0;
			for (index i = 0; i < my_size + 1; i++)
			{
				if ((1u << (cur + 1)) < i)
					cur++;
				my_log[i] = cur;
			}
			table = new Element*[my_log[my_size] + 1];
			for (index i = 0; i <= my_log[my_size]; i++)
			{
				table[i] = new Element[my_size];
			}
			for (index i = 0; i < my_size; i++)
			{
				table[0][i] = *begin;
				begin++;
			}
			for (index i = 1; i <= my_log[my_size]; i++)
			{
				for (index j = 0; j + (1 << i) - 1 < my_size; j++)
				{
					table[i][j] = func(table[i - 1][j], table[i - 1][j + (1 << (i - 1))]);
				}
			}
		}/*}}}*/

		Element get(index l, size_t r) const /*{{{*/
		{
			return func(table[my_log[r - l]][l], table[my_log[r - l]][r - (1 << my_log[r - l])]);
		}/*}}}*/

		size_t size() const/*{{{*/
		{
			return my_size;
		}/*}}}*/
};/*}}}*/

template<class Element> class MinQueue/*{{{*/
{
	private:
		std::vector<std::pair<Element, Element>> left;
		std::vector<Element> right;
		std::function<bool (Element, Element)> func;
	public:
		MinQueue(std::function<bool (Element, Element)> new_func): func(new_func){};

		size_t size() const/*{{{*/
		{
			return left.size() + right.size();
		}/*}}}*/

		Element get_min() const/*{{{*/
		{
			assert(size() > 0);
			if (left.size() == 0)
				return right.back();
			else if (right.size() == 0)
				return left.back().second;
			else if(func(left.back().second, right.back()))
				return left.back().second;
			else
				return right.back();
		}/*}}}*/

		void pop_front()/*{{{*/
		{
			assert(size() > 0);
			if (right.size() == 0)
			{
				while (left.size())
				{
					right.push_back(right.size() ? (func(left.back().first, right.back()) ? left.back().first : right.back()) : left.back().first);
					left.pop_back();
				}
			}
			right.pop_back();
		}/*}}}*/

		void push(const Element &a)/*{{{*/
		{
			left.push_back({a, (left.size() ? (func(left.back().second, a) ? left.back().second : a) : a)});
		}/*}}}*/

};/*}}}*/

template<unsigned int MOD> class RingElement/*{{{*//*{{{*/
{

	private:/*{{{*/
		constexpr static unsigned long long find_phi(unsigned long long p)/*{{{*/
		{
			unsigned long long res = p;
			for (unsigned long long i = 2; i * i <= p; i++)
			{
				if (p % i == 0)
				{
					while (p % i == 0)
						p /= i;
					res -= res / i;
				}
			}
			if (p > 1)
			{
				res -= res / p;
			}
			return res;
		}/*}}}*/
		unsigned int me;
		const static unsigned int PHI = find_phi(MOD);
		static RingElement<MOD> pow(RingElement<MOD> a, unsigned long long b)/*{{{*/
		{
			b = b % PHI;
			RingElement<MOD> res = 1;
			while (b > 0)
			{
				if (b % 2 == 0)
				{
					a = a * a;
					b /= 2;
				}
				else
				{
					res = res * a;
					b--;
				}
			}
			return res;
		}/*}}}*/

		static unsigned int gcd(unsigned int a, unsigned int b)/*{{{*/
		{
			while (a != 0)
			{
				unsigned int c = a;
				a = a % b;
				b = c;
			}
			return b;
		}/*}}}*//*}}}*/

	public:/*{{{*/
		RingElement(unsigned long long new_me): me(new_me % MOD){}
		RingElement(): me(0){}

	unsigned long long get_phi() const/*{{{*/
	{
		return PHI;
	}/*}}}*/

	friend std::istream &operator >>(std::istream &a, RingElement<MOD> &b)/*{{{*/
	{
		a >> b.me;
		b.me = b.me % MOD;
		return a;
	}/*}}}*/

	friend std::ostream &operator <<(std::ostream &a, const RingElement<MOD> &b)/*{{{*/
	{
		a << b.me;
		return a;
	}/*}}}*/

	friend RingElement<MOD> operator +(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
		return {a.me + b.me >= MOD ? a.me + b.me - MOD : a.me + b.me};
	}/*}}}*/

	friend RingElement<MOD> operator -(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
		return {a.me >= b.me ? a.me - b.me : a.me - b.me + MOD};
	}/*}}}*/

	friend RingElement<MOD> operator *(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
		return {(unsigned long long)a.me * b.me};
	}/*}}}*/

	friend RingElement<MOD> operator /(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
#ifdef MORE_ASSERTS
		assert(gcd(b.me, MOD) == 1);
#endif
		return a.me * pow(b, PHI - 1);
	}/*}}}*/

	RingElement<MOD> operator++()/*{{{*/
	{
		me++;
		if (me == MOD)
			me = 0;
		return *this;
	}/*}}}*/

	RingElement<MOD> &operator++(int)/*{{{*/
	{
		RingElement<MOD> cp = *this;
		me++;
		if (me == MOD)
			me = 0;
		return cp;
	}/*}}}*/

	RingElement<MOD> operator--()/*{{{*/
	{
		me--;
		if (me > MOD)
			me = MOD - 1;
		return *this;
	}/*}}}*/

	RingElement<MOD> &operator--(int)/*{{{*/
	{
		RingElement<MOD> cp = *this;
		me--;
		if (me > MOD)
			me = MOD - 1;
		return cp;
	}/*}}}*/

	RingElement<MOD> &operator +=(const RingElement<MOD> &a)/*{{{*/
	{
		me = (*this + a).me;
	}/*}}}*/

	RingElement<MOD> &operator -=(const RingElement<MOD> &a)/*{{{*/
	{
		me = (*this - a).me;
	}/*}}}*/

	RingElement<MOD> &operator *=(const RingElement<MOD> &a)/*{{{*/
	{
		me = (*this * a).me;
	}/*}}}*/

	RingElement<MOD> &operator /=(const RingElement<MOD> &a)/*{{{*/
	{
		me = (*this / a).me;
	}/*}}}*/

	friend bool operator ==(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
		return a.me == b.me;
	}/*}}}*/

	friend bool operator !=(const RingElement<MOD> &a, const RingElement<MOD> &b)/*{{{*/
	{
		return a.me != b.me;
	}/*}}}*//*}}}*/

};
/*}}}*/

template<unsigned long MOD>/*{{{*/
RingElement<MOD> pow(RingElement<MOD> a, unsigned long long b)
{
	b = b % a.get_phi();
	RingElement<MOD> res = 1;
	while (b > 0)
	{
		if (b % 2 == 0)
		{
			a = a * a;
			b /= 2;
		}
		else
		{
			res = res * a;
			b--;
		}
	}
	return res;
}/*}}}*//*}}}*/

using namespace std;

bool check(const string &s)
{
	for (index i = 0; i < s.size(); i++)
	{
		if (s[i] != s[s.size() - 1 - i])
			return false;
	}
	return true;
}

int main()
{
#ifdef ONPC
	assert(freopen("a.in", "r", stdin));
	assert(freopen("a.out", "w", stdout));
#else
#endif
	string s;
	cin >> s;
	string clr;
	for (auto i: s)
		if (i != 'x')
			clr += i;
	if (!check(clr))
	{
		cout << -1 << endl;
		return 0;
	}
	index posl = (index)-1, posr = (index)-1;
	if (clr.size() % 2 == 1)
	{
		size_t cnt = (size_t)-1;
		for (index i = 0; i < s.size(); i++)
		{
			if (s[i] != 'x')
				cnt++;
			if (cnt == clr.size() / 2)
			{
				posl = i, posr = i;
				break;
			}
		}
	}
	else
	{
		size_t cnt = (size_t)-1;
		for (index i = 0; i < s.size(); i++)
		{
			if (s[i] != 'x')
			{
				cnt++;
				if (cnt == clr.size() / 2 - 1)
				{
					posl = i;
				}
				if (cnt == clr.size() / 2)
				{
					posr = i;
				}
			}
		}
	}
	size_t ans = 0;
	while (posl != (index)-1 or posr < s.size())
	{
		if (posl != (index)-1 and posr < s.size())
		{
			if (s[posl] == s[posr])
			{
				posl--;
				posr++;
			}
			else if (s[posl] == 'x')
			{
				posl--;
				ans++;
			}
			else 
			{
				posr++;
				ans++;
			}
		}
		else
		{
			if (posl == (index)-1)
			{
				posr++;
				ans++;
			}
			else
			{
				posl--;
				ans++;
			}
		}
	}
	cout << ans << endl;
}


Measured memory_bytes: 640