#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll n, d;
	cin >> n >> d;

	vector<ll> t(n), k(n);
	rep(i, n) cin >> t[i] >> k[i];

	ll dp[n][2];
	rep(i, n) rep(j, 2) dp[i][j] = 0;

	dp[0][0] = t[0];
	dp[0][1] = k[0] - d;

	rep(i, n-1){
		dp[i+1][0] = max(dp[i][0] + t[i+1], dp[i][1] + t[i+1] - d);
		dp[i+1][1] = max(dp[i][0] + k[i+1] - d, dp[i][1] + k[i+1]);
	}

	cout << max(dp[n-1][0], dp[n-1][1]) << endl;
}