#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
#define REP(i,x,n) for(int i=x;i<n;i++)
#define ALL(v) (v).begin(),(v).end()
#define MP(a,b) make_pair(a,b)
typedef long long LL;
typedef pair<int, int> PI;
typedef vector<int> VI;
const LL MOD = 1000000007LL;
int v[101][2];
int dp[101][2];
int main() {
	int N, D;
	cin >> N >> D;
	for(int i=1;i<=N;i++) {
		cin >> v[i][0] >> v[i][1];
	}
	dp[0][1] = -1;
	for (int i = 1; i <= N; i++) {
		//j=0
		if(dp[i-1][0]!=-1) dp[i][0] = dp[i - 1][0] + v[i][0];
		if(dp[i-1][1]!=-1) dp[i][0]=max(dp[i][0],dp[i - 1][1] + v[i][0] - D);
		//j=1
		if (dp[i - 1][1] != -1) dp[i][1] = dp[i - 1][1] + v[i][1];
		if (dp[i - 1][0] != -1) dp[i][1] = max(dp[i][1], dp[i - 1][0] + v[i][1] - D);
	}
	cout << max(dp[N][0], dp[N][1]) << endl;
}