#include <bits/stdc++.h>

using namespace std;

#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

#define ll long long
#define ld long double
#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define FOR(i,l,r) for(int i=(l);i<=(r);i++)
#define ROF(i,r,l) for(int i=(r);i>=(l);i--)
int inf=1000000007;
ll infl=1000000000000000007;
ll mod=1000000007;
ll mod1=998244353;

const int N=200007;

bool is[N];

int n;

void case0()
{
	bool ok=1;
	FOR(i,1,n) if(!is[i]) ok=0;
	if(!ok) cout<<-1<<endl;
	else cout<<0<<endl;
}
void case01()
{
	int ans=0;
	FOR(i,2,n) if(is[i]&&!is[i-1]) ans++;
	cout<<ans<<endl;
}
void case00()
{
	int ile=0,ans=0;
	FOR(i,1,n) 
	{
		if(!is[i]) ile++;
		if(i>1&&!is[i]&&!is[i-1]) ans++;
	}
	if(ile>n/2&&n!=1) cout<<-1<<endl;
	else cout<<ans<<endl;
}

signed main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	string s,t;
	cin>>s>>t;
	n=sz(s);
	FOR(i,0,n-1) is[i+1]=s[i]-'0';
	if(t[0]=='1')
	{
		FOR(i,1,n) is[i]^=1;
		FOR(i,0,sz(t)-1)
		{
			if(t[i]=='0') t[i]='1';
			else t[i]='0';
		}
	}
	if(t=="0") case0();
	if(t=="01") case01();
	if(t=="00") case00();
	
	
	
	return 0;
}