#include <iostream>
#include <vector>

using namespace std;

int main()
{
	int N, D;

	cin >> N >> D;
	int T[101], K[101];
	int dp[101][2];

	for( int i = 1; i <= N; i++ ) {
		cin >> T[i] >> K[i];
	}

	dp[0][0] = 0;
	dp[0][1] = -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][1] + K[i], dp[i-1][0] + K[i] - D );
	}

	/*
	for( int i = 1; i <= N; i++ ) {
	   cout << "i " << i << " T=" << dp[i][0] << " K=" << dp[i][1] << endl;
	}
	/**/

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


	return 0;

}