#include <iostream>
#include <algorithm>
#include <vector>
#include <iomanip>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
const int INF = (int)1e9 + 1;

int N;
ll D, C[101][2];
ll dp[101][2];
//0:tokyo 1:kyoto

int main(void) {
	cin >> N >> D;
	
	for (int i = 0; i < N; i++) {
		cin >> C[i][0] >> C[i][1];
	}

	for (int i = 0; i <= N; i++) {
		for (int j = 0; j < 2; j++) {
			dp[i][j] = -INF;
		}
	}

	dp[0][0] = 0;
	dp[0][1] = -D;

	for (int i = 0; i < N; i++) {
		for (int j = 0; j < 2; j++) {
			dp[i + 1][j] = max(dp[i + 1][j], dp[i][j] + C[i][j]);
			dp[i + 1][1 - j] = max(dp[i + 1][1 - j], dp[i][j] + C[i][1 - j] - D);
		}
	}

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