#include<bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define pb push_back
int dy[]={0, 0, 1, -1, 1, 1, -1, -1};
int dx[]={1, -1, 0, 0, 1, -1, -1, 1};

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define mp make_pair
#define fi first
#define sc second
ll n,d;
ll t[200],k[200];
bool done[201][2];
ll memo[201][2];

ll solve(ll i,ll j) {
	if(done[i][j]) {
		return memo[i][j];
	}
	if(i >= n) {
		return 0;
	}

	if(j == 0) {
		memo[i][j] = max(t[i] + solve(i + 1,0),k[i] + solve(i + 1,1) - d);
		done[i][j] = true;
		return memo[i][j];
	}
	if(j == 1) {
		memo[i][j] = max(t[i] + solve(i + 1,0) - d,k[i] + solve(i + 1,1));
		done[i][j] = true;
		return memo[i][j];
	}
}

int main(){
	cin >> n;
	cin >> d;
	REP(i,n) {
		cin >> t[i] >> k[i];
	}

	cout << solve(0,0) << endl;
	return 0;
}