#include <bits/stdc++.h>   using namespace std;   int main() {     int w, h, h1, w1, h2, w2;     cin >> w >> h >> w1 >> h1 >> w2 >> h2;     for (int i = h; i >= 0; i--) {       w += i;       if (i == h1) w -= w1;       if (i == h2) w -= w2;       if (w < 0) w = 0;     }     cout << w << endl;     return 0;   }   