#include<iostream>
#include<string.h>

using namespace std;

int dp[100+1][2];

int main(){
	memset(dp,0,sizeof(dp));
	int n,d;
	cin >> n >> d;
	int t[100],k[100];
	for(int i = 0;i < n;i++) cin >> t[i] >> k[i];
	
	for(int i = 0;i < n;i++){
		if(i == 0){
			dp[i+1][0] = t[i];
			dp[i+1][1] = k[i] - d;
		}else{
			dp[i+1][0] = max(dp[i][0]+t[i],dp[i][1]+t[i]-d);
			dp[i+1][1] = max(dp[i][0]+k[i]-d,dp[i][1]+k[i]);
		}
	}
	
	cout << max(dp[n][0],dp[n][1]) << endl;
	return 0;
}