#include <bits/stdc++.h>
#define rep(i,j,k) for(int i=(j);i<=(k);i++)
#define per(i,j,k) for(int i=(j);i>=(k);i--)
#define uint unsigned int
#define ll long long
#define ull unsigned long long
#define db double
#define ldb long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define mkp make_pair
#define eb emplace_back
#define SZ(S) (int)S.size()
//#define mod 998244353
//#define mod 1000000007
#define inf 0x3f3f3f3f
#define linf 0x3f3f3f3f3f3f3f3f
using namespace std;

template <class T>
void chkmax(T &x,T y){
	if(x < y) x = y;
}

template <class T>
void chkmin(T &x,T y){
	if(x > y) x = y;
}

inline int popcnt(int x){
	return __builtin_popcount(x);
}

inline int ctz(int x){
	return __builtin_ctz(x);
}


/*ll power(ll p,int k = mod - 2){
	ll ans = 1;
	while(k){
		if(k % 2 == 1) ans = ans * p % mod;
		p = p * p % mod;
		k /= 2;	
	}
	return ans;
}*/
int n,type;
char s[200005],t[5];

int solve1(char ch){
	rep(i,1,n) if(s[i] == ch) return -1;
	return 0;
}

void rev(){
	rep(i,1,n){
		if(s[i] == '0') s[i] = '1';
		else s[i] = '0';
	}
	rep(i,1,type){
		if(t[i] == '0') t[i] = '1';
		else t[i] = '0';
	}
}

int solve2dif(){
	int cnt = 1;
	rep(i,2,n) if(s[i] != s[i - 1]) cnt++;
	if(s[1] == '0') return (cnt - 1) / 2;
	return cnt / 2;
}

int solve2sam(){
	int cnt = 0,ans = 0;
	rep(i,1,n){
		if(s[i] == '0') cnt--;
		else cnt++;
	}
	if(cnt < -1) return -1;
	rep(i,2,n) if(s[i] == '0' && s[i - 1] == '0') ans++;
	return ans;
}

int main(){
	scanf("%s",s + 1);
	n = strlen(s + 1);
	scanf("%s",t + 1);
	type = strlen(t + 1);
	if(type == 1){
		printf("%d\n",solve1(t[1]));
		return 0;
	}else if(type == 2){
		if(t[1] != t[2]){
			if(t[1] == '0') rev();
			printf("%d\n",solve2dif());
		}else{
			if(t[1] == '1') rev();
			printf("%d\n",solve2sam());
		}
	}
	return 0;
}