#include <iostream>
#include<bits/stdc++.h>
using namespace std;

int main() {
	// your code goes here
	string s1,s2;
	cin>>s1>>s2;
	
	int rr1=0;
	int rr2=0;
	int ww1=0;
	int ww2=0;
	int q1=0;
	int q2=0;
	
	int ans=0;
	
	
	int n = s1.size();
	for(int i=0;i<n;i++){
		if(s1[i]=='R'){
			if(s2[i]=='R'){
				cout<<-1<<endl;
				return 0;
			}
			rr1++;
		}
		else if(s1[i]=='W'){
			if(s2[i]=='W'){
				cout<<-1<<endl;
				return 0;
			}
			ww1++;
		}
		else{
			if(s2[i]=='R'){
				s1[i]='W';
				
				ww1++;
				ans++;
				
			}
			else if(s2[i]=='W'){
				s1[i]='R';
				rr1++;
				ans++;
			}
			else{
				q1++;
			}
		}
	}
	
for(int i=0;i<n;i++){
		if(s2[i]=='R'){
			rr2++;
		}
		else if(s2[i]=='W'){
			ww2++;
		}
		else{
			if(s1[i]=='R'){
				ww2++;
				ans++;
			}
			else if(s1[i]=='W'){
				rr2++;
				ans++;
			}
			else{
				q2++;
			}
		}
	}
	
	int diff1 = abs(rr1-ww1);
	int diff2 = abs(rr2-ww2);
	if(q1>=diff1){
		ans = ans + diff1;
	}
	else{
		cout<<-1<<endl;
		return 0;
	}
	
	if(q2>=diff2){
		ans = ans + diff2;
	}
	else{
		cout<<-1<<endl;
		return 0;
		
		
	}
	
	cout<<ans<<endl;
	
	return 0;
}