#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>
#include<iterator>

#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define rep(i,n) for(int i=0; i<n; i++)
#define FOR(i,a,n) for(int i=a; i<n; i++)

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ll, char> plc;
typedef pair<string, string> pss;

int n, d;
int t[110], k[110];
ll dp[110][110];

int main()
{
	cin >> n >> d;
	rep(i, n) 
		cin >> t[i+1] >> k[i+1];

	dp[0][1] = -d;
	
	FOR(i, 1,n+1) {
		dp[i][0] = max(dp[i-1][0] + t[i],
			dp[i-1][1] + k[i] - d);
		dp[i][1] = max(dp[i-1][0] + t[i] -d,
			dp[i-1][1] + k[i]);
	}

	cout << max(dp[n][0], dp[n][1]) << endl;;
	return 0;
}