#include<iostream>
#include<algorithm>
#include<functional>
#include<cmath>
#include<string>
#include<vector>
#include<queue>
using namespace std;
#define ll long long
const int mod = 1000000007;
const ll INF = 1000000000000000000;

int T[110],K[110];
ll dp[110][2];
int main()
{
	int N, D;
	cin >> N >> D;
	for (int i = 0; i < N; i++) {
		cin >> T[i] >> K[i];
	}
	dp[0][0] = T[0];
	dp[0][1] = K[0] - D;
	for (int i = 1; i < N; i++) {
		dp[i][0] = max(dp[i - 1][0] + T[i], dp[i - 1][1] + T[i] - D);
		dp[i][1] = max(dp[i - 1][0] + K[i] - D, dp[i - 1][1] + K[i]);
	}
	cout << max(dp[N - 1][0], dp[N - 1][1]) << endl;
}