#include <bits/stdc++.h>
using namespace std;
long long t1,d1,t2,d2,t,d;
int mx=-1;
int main(){
	cin>>t>>d>>t1>>d1>>t2>>d2;
	if(d>d1 and d>d2){
		cout<<"-1";
		return 0;
	} 
	if(t>t1 or t>t2){
        cout<<"0";
        return 0;
    }
    if(t1>t2) mx=t2;
    else if(t2>t1) mx=t1;
    else mx=t1;
    cout<<t-mx;
	return 0;
}