// Mahmud Isali
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
// #include <bits/extc++.h>
using namespace std;
// using namespace __gnu_pbds;
// #define int long long
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pb push_back
// #define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
const long long INF = 1e18, MOD = 1e9 + 9, N = 5e5 + 10;
void solve() {
int n,k;
cin >> n >> k;
int ans = 0;
for(int i = 1; i <= n; i++) {
int l,r;
cin >> l >> r;
ans = max(ans, r - l);
}
cout << ans << endl;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int t = 1;
// cin >> t;
for(int T = 1; T <= t; T++) {
solve();
}
}
