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;
}