#include <bits/stdc++.h>
using namespace std;
string s1,s2;
int ans,s[1010],s3,cnt;
int main(){
    cin>>s1>>s2;
    for(int i=0;i<s1.size();i++){
    	if(s1[i]!=s2[i]){
    		s3++;
    	}
    	if((s1[i]=='o'||s2[i]=='o')&&s1[i]!=s2[i]){
    		s[++cnt]=i;
    	}
    }
    if(s3%2==1){
    	cout<<-1;
    	return 0;
    }
    for(int i=2;i<=cnt;i+=2){
    	ans+=s[i]-s[i-1];
    }
    cout<<ans;
    return 0;
}