CF A

#include<bits/stdc++.h>
#include<cmath>
using namespace std;
int main()
{
    int w,h,u1,u2,h1,h2,a[110],i,j;
    for(i=0;i<=100;i++){
        a[i]=0;
    }
    cin>>w>>h;
    cin>>u1>>h1;
    a[h1]=u1;
    cin>>u2>>h2;
    a[h2]=u2;
    for(i=h;i>=0;i--){
        w+=i;
        w-=a[i];
        if(w<0)
            w=0;
    }
    cout<<w<<endl;
    return 0;
}