https://codeforces.com/contest/1099/problem/A

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,h,w,k,q,ans;
int a;
char str;
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    scanf("%d%d",&w,&h);
    scanf("%d%d",&n,&m);
    scanf("%d%d",&k,&q);
    if(m<q){
        swap(k,n);
        swap(m,q);
    }
    ans=w;
    while(h>=0){
        ans+=h;
        if(h==m){
            ans=max(ans-n,0);
        }else if(q==h){
            ans=max(ans-k,0);
        }
        h--;

    }
    cout << ans << endl;
    //cout << "Hello world!" << endl;
    return 0;
}