#include<bits/stdc++.h>
using namespace std;
namespace my_std{
	#define ll long long
	#define bl bool
	ll my_pow(ll a,ll b,ll mod){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res=(res*a)%mod;
			a=(a*a)%mod;
			b>>=1;
		}
		return res;
	}
	ll qpow(ll a,ll b){
		ll res=1;
		if(!b) return 1;
		while(b){
			if(b&1) res*=a;
			a*=a;
			b>>=1;
		}
		return res;
	}
	#define db double
	#define pf printf
	#define pc putchar
	#define fr(i,x,y) for(register ll i=(x);i<=(y);i++)
	#define pfr(i,x,y) for(register ll i=(x);i>=(y);i--)
	#define go(u) for(ll i=head[u];i;i=e[i].nxt)
	#define enter pc('\n')
	#define space pc(' ')
	#define fir first
	#define sec second
	#define MP make_pair
	#define il inline
	#define inf 8e18
	#define random(x) rand()*rand()%(x)
	#define inv(a,mod) my_pow((a),(mod-2),(mod))
	il ll read(){
		ll sum=0,f=1;
		char ch=0;
		while(!isdigit(ch)){
			if(ch=='-') f=-1;
			ch=getchar();
		}
		while(isdigit(ch)){
			sum=sum*10+(ch^48);
			ch=getchar();
		}
		return sum*f;
	}
	il void write(ll x){
		if(x<0){
			x=-x;
			pc('-');
		}
		if(x>9) write(x/10);
		pc(x%10+'0');
	}
	il void writeln(ll x){
		write(x);
		enter;
	}
	il void writesp(ll x){
		write(x);
		space;
	}
}
using namespace my_std;
ll n,m;
char s[200020],t[4];
int main(){
	scanf("%s",s+1);
	scanf("%s",t+1);
	n=strlen(s+1);
	m=strlen(t+1);
	if(t[1]=='1'){
		fr(i,1,n) s[i]^=1;
		fr(i,1,m) t[i]^=1;
	}
	if(m==1){
		bl ck=0;
		fr(i,1,n) if(s[i]=='0') ck=1;
		if(!ck) write(0);
		else write(-1);
		return 0;
	}
	if(m==2){
		if(t[2]=='1'){
			ll cnt=0;
			fr(i,1,n-1) if(s[i]=='0'&&s[i+1]=='1') cnt++;
			write(cnt);
		}
		else{
			ll cnt0=0,cnt1=0,cnt=0;
			fr(i,1,n){
				if(s[i]=='0') cnt0++;
				if(s[i]=='1') cnt1++;
				if(i<n&&s[i]=='0'&&s[i+1]=='0') cnt++;
			}
			if(cnt1<(cnt0-1)) write(-1);
			else write(cnt);
		}
		return 0;
	}
	if(t[2]=='1'&&t[3]=='1'){
		reverse(s+1,s+n+1);
		reverse(t+1,t+m+1);
		fr(i,1,n) s[i]^=1;
		fr(i,1,m) t[i]^=1;
	}
	if(t[2]=='1'){
		ll cnt=0;
		fr(i,2,n-1) if(s[i]=='1'&&s[i-1]=='0'&&s[i+1]=='0') cnt++;
		write((cnt+1)/2);
		return 0;
	}
	if(t[3]=='1'){
		ll lst=0,cnt=0;
		fr(i,1,n){
			if(s[i]=='1'){
				if(lst<(i-2)) cnt++;
				lst=i;
			}
		}
		write(cnt);
		return 0;
	}
	ll lst=0,cnt0=0,cnt1=0,cnt=0;
	fr(i,1,n){
		if(s[i]=='0') cnt0++;
		else{
			cnt1++;
			cnt+=(i-lst-2)/2;
			lst=i;
		}
	}
	cnt+=(n-lst-1)/2;
	if((cnt0-1)/2>cnt1) write(-1);
	else write(cnt);
}