#include<iostream>
#include<cstdio>
#include<algorithm>
#define N 200000
using namespace std;
int read()
{
    char c=0;
    int sum=0;
    while (c<'0'||c>'9') c=getchar();
    while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
    return sum;
}
int n,res;
char c[N+1];
string s,t;
int main()
{
    int cnt=0;
    cin>>s>>t,n=s.length();
    for (int i=1;i<=n;++i) c[i]=s[i-1];
    if (t[0]=='1')
    {
	for (int i=1;i<=n;++i) c[i]^=('0'^'1');
	for (int i=0;i<t.length();++i) t[i]^=('0'^'1');
    }
    if (t=="001")
    {
	reverse(c+1,c+n+1);
	for (int i=1;i<=n;++i) c[i]^=('0'^'1');
	for (int i=0;i<t.length();++i) t[i]^=('0'^'1');
    }
    if (t=="0")
    {
	for (int i=1;i<=n;++i)
	    if (c[i]=='0')
	    {
		puts("-1");
		return 0;
	    }
	puts("0");
    }
    else if (t=="00")
    {
	for (int i=1;i<=n;++i)
	    if (c[i]=='0')
		cnt++;
	if (cnt>((n+1)>>1))
	{
	    puts("-1");
	    return 0;
	}
	for (int i=1;i<=n-1;++i)
	    if (c[i]=='0'&&c[i+1]=='0')
		res++;
	printf("%d\n",res);
    }
    else if (t=="01")
    {
	for (int i=1;i<=n-1;++i)
	    if (c[i]=='0'&&c[i+1]=='1')
		res++;
	printf("%d\n",res);
    }
    else if (t=="000")
    {
	for (int i=1;i<=n;++i)
	    if (c[i]=='0')
		cnt++;
	if (cnt>((n/3)<<1)+n%3)
	{
	    puts("-1");
	    return 0;
	}
	cnt=0;
	for (int i=1;i<=n;++i)
	{
	    if (c[i]=='0') cnt++;
	    if (c[i]=='0'&&(i==n||c[i+1]=='1')) res+=((cnt-1)>>1);
	}
	printf("%d\n",res);
    }
    else if (t=="010")
    {
	for (int i=1;i<=n-2;++i)
	    if (c[i]=='0'&&c[i+1]=='1'&&c[i+2]=='0')
		res++;
	printf("%d\n",(res+1)>>1);
    }
    else
    {
	for (int i=1;i<=n-2;++i)
	    if (c[i]=='0'&&c[i+1]=='1'&&c[i+2]=='1')
		res++;
	printf("%d\n",res);
    }
    return 0;
}