#include <bits/stdc++.h>
using namespace std;

#define TASK "text"
#define ll long long
#define endl "\n"
#define FOR(i, k, n) for (int i = (k), _n = (n); i <= _n; i++)
#define FOV(i, k, n) for (int i = (k), _n = (n); i >= _n; i--)
#define REP(i, n) FOR(i, 1, n)
#define REV(i, n) FOV(i, n, 1)
#define MASK(i) (1LL << (i))
#define ON(i, n) (i | MASK(n))
#define OFF(i, n) (i & ~MASK(n))
#define BIT(i, n) (i & MASK(n))
#define POP_CNT(i) (__builtin_popcountll(i))
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pil pair<int, ll>
#define pli pair<ll, int>
#define fi first
#define se second
#define pb push_back

const int MAXN = 2e5 + 10;
int n, q;
ll res, a[MAXN], d[MAXN];

int main(){
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    if (fopen(TASK".inp", "r")){
        freopen(TASK".inp", "r", stdin);
        freopen(TASK".out", "w", stdout);
        freopen(TASK".log", "w", stderr);
    }

    cin >> n >> q;

    REP (i, q){
        int l, r;
        cin >> l >> r;
        FOR (j, l, r) a[j] += j - l + 1;
    }

    REP (i, n) res = max(res, a[i]);

    cout << res;

    cerr << endl << "RUNTIME: " << 1.0 * clock() / CLOCKS_PER_SEC;
    return 0;
}