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

int main() {
	int N, cost;
	cin >> N >> cost;
	vector<int>tokyo(N);
	vector<int>kyoto(N);
	for (int i = 0; i < N; i++) {
		cin >> tokyo[i] >> kyoto[i];
	}
	vector<int>dp_tokyo(N+1,0);
	vector<int>dp_kyoto(N + 1,0);
	dp_kyoto[0] = -cost;
	for (int i = 0; i < N; i++) {
		dp_tokyo[i + 1] = max(dp_tokyo[i] + tokyo[i], dp_kyoto[i] - cost + tokyo[i]);
		dp_kyoto[i + 1] = max(dp_kyoto[i] + kyoto[i], dp_tokyo[i] - cost + kyoto[i]);
	}
	if (dp_tokyo[N] > dp_kyoto[N]) {
		cout << dp_tokyo[N] << endl;
	}
	else {
		cout << dp_kyoto[N] << endl;
	}
	return 0;
}