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

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,int> P;
typedef pair<ll,P> PP;

//-----------------------------------------------------------------------------

int n,d;
ll dp[110][2];

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>n>>d;
	dp[0][0]=0,dp[0][1]=-INF;
	REP(i,n) {
		int t,k;cin>>t>>k;
		dp[i+1][0]=max(dp[i][0]+t,dp[i][1]-d+t);
		dp[i+1][1]=max(dp[i][0]-d+k,dp[i][1]+k);
	}
	cout<<max(dp[n][0],dp[n][1])<<endl;

	return 0;
}