#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <string>

using namespace std;

const int inf = 1e9;

int main() {
	int n, d;
	cin >> n >> d;

	vector<int> t(n+1, 0), k(n+1, 0);
	for (int i = 1; i <= n; i++) {
		cin >> t[i] >> k[i];
	}

	vector<vector<int>> dp(n + 1, vector<int>(2, 0));

	dp[0][1] = -inf;

	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][1] + k[i], dp[i-1][0] + k[i] - d);

		// printf("dp[%d][0] = %d: %d %d\n", i, dp[i][0], dp[i-1][0] + t[i], dp[i-1][1] + k[i] - d);
		// printf("dp[%d][1] = %d: %d %d\n", i, dp[i][1], dp[i-1][1] + k[i], dp[i-1][0] + t[i] - d);

	}

	// for (int i = 0; i <= n; i++) {
	// 	printf("%2d: %10d %10d\n", i, dp[i][0], dp[i][1]);
	// }

	int ans = max(dp[n][0], dp[n][1]);
	cout << ans << endl;
	return 0;
}