#include <iostream>
#include <algorithm>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<iomanip>
#include<ctime>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<bitset>
#include<cassert>
#define sqr(x) ((x)*(x))
#define fz1(i,n) for ((i)=1;(i)<=(n);(i)++)
#define fd1(i,n) for ((i)=(n);(i)>=1;(i)--)
#define fz0g(i,n) for ((i)=0;(i)<=(n);(i)++)
#define fd0g(i,n) for ((i)=(n);(i)>=0;(i)--)
#define fz0k(i,n) for ((i)=0;(i)<(n);(i)++)
#define fd0k(i,n) for ((i)=(((long long)(n))-1);(i)>=0;(i)--)
#define fz(i,x,y) for ((i)=(x);(i)<=(y);(i)++)
#define fd(i,y,x) for ((i)=(y);(i)>=(x);(i)--)
#define fzin fz1(i,n)
#define fzim fz1(i,m)
#define fzjn fz1(j,n)
#define fzjm fz1(j,m)
#define ff(c,itr) for (__typeof((c).begin()) itr=(c).begin();itr!=(c).end();++itr)
#define pb push_back
#define mk make_pair
#define rdst(st,len){static char ss[len];scanf(" %s",ss);(st)=ss;}
#define spln(i,n) (i==n?'\n':' ')
#define fac_init(n){fac[0]=fac[1]=inv[1]=fi[0]=fi[1]=1;fz(i,2,n){fac[i]=1ll*fac[i-1]*i%mod;inv[i]=1ll*(mod-mod/i)*inv[mod%i]%mod;fi[i]=1ll*fi[i-1]*inv[i]%mod;}}
using namespace std;
typedef long long i64;
typedef long double f80;
typedef unsigned int u32;
typedef unsigned long long u64;
//typedef __int128 i128;
//typedef unsigned __int128 u128;
#ifndef ONLINE_JUDGE
	FILE *___=freopen("1.in","r",stdin);
#endif
inline void read(int &x)
{
	char c;int f=1;
	while(!isdigit(c=getchar()))if(c=='-')f=-1;
	x=(c&15);while(isdigit(c=getchar()))x=(x<<1)+(x<<3)+(c&15);
	x*=f;
}
int n,m,i,j;
string s,t;
int main()
{
	cin>>s>>t;
	if(t[0]=='1'){
		fz0k(i,s.size())s[i]^=1;
		fz0k(i,t.size())t[i]^=1;
	}
	if(t.size()==1){
		if(s.find(t)!=string::npos) puts("-1");
		else puts("0");
		return 0;
	}
	if(t.size()==2){
		if(t[0]!=t[1]){
			int res=0;
			fz1(i,s.size()-1)if(s[i-1]=='0'&&s[i]=='1')res++;
			printf("%d\n",res);
		}
		else{
			s="1"+s+"1";int c0=0,c1=0;
			fz1(i,s.size()-1)if(s[i-1]=='0'&&s[i]=='0')c0++;else if(s[i-1]=='1'&&s[i]=='1')c1++;
			if(c0>c1)puts("-1"); else printf("%d\n",c0);
		}
		return 0;
	}
	if(t[0]!=t[1]) reverse(t.begin(),t.end()),reverse(s.begin(),s.end());
	if(t[0]=='1'){
		fz0k(i,s.size())s[i]^=1;
		fz0k(i,t.size())t[i]^=1;
	}
	if(t[0]!=t[2]){
		int res=0;i=s.size()-1;
		while(i>=0&&s[i]=='0')i--;
		while(i>=0){
			while(i>=0&&s[i]=='1')i--;
			if(i<0) break;i--;
			if(i>=0&&s[i]=='0')i--,res++;
			while(i>=0&&s[i]=='0')i--;
		}
		printf("%d\n",res);
		return 0;
	}
	if(t[0]!=t[1]){
		int c=0;
		fz1(i,((int)s.size())-2)if(s[i-1]==t[0]&&s[i]==t[1]&&s[i+1]==t[2]) c++;
		printf("%d\n",(c+1)/2);
		return 0;
	}
	int res=0,lst=0,c=0;
	fz0k(i,s.size()){
		if(s[i]=='0')lst++;
		else{
			if(lst==0) c+=2; else res+=(lst-1)/2,c+=(lst%2);
			lst=0;
		}
	}
	if(lst==0) c+=2; else res+=(lst-1)/2,c+=(lst%2);
	c/=2;if(res>c)puts("-1");else printf("%d\n",res);
	return 0;
}