#include<iostream>
#include<vector>
#include<algorithm>
#include<cctype>
#include<utility>
#include<string>
#include<cstring>
#include<cmath>
#include<numeric>
#include<queue>
#include<climits>
#include<cstdio>


#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)
//#define INF ((LLONG_MAX) / (2))
using namespace std;
typedef pair<int, int> P;
typedef pair<llong, llong> LP;
typedef pair<int, P> PP;
typedef pair<llong, LP> LPP;
typedef long long int ll;
typedef pair<ll,int> LL_IP;
typedef pair<ll,ll> LL_LLP;
#define INF 1e9+7
#define N_MAX 105
int dp[2][N_MAX];

int main(){

    int n,d;
    cin >> n >> d;
    vector<int> t(n),k(n);
    REP(i,n){
        cin >> t[i] >> k[i];
    }
    bool now = true;//true:東京、false:京都
    
    dp[0][0] = t[0];
    dp[1][0] = k[0]-d;
    for(int i = 1;i < n;i++){
        dp[0][i] = max(dp[0][i-1]+t[i],dp[1][i-1]+t[i]-d);
        dp[1][i] = max(dp[0][i-1]+k[i]-d,dp[1][i-1]+k[i]);
    }
    int ans = max(dp[0][n-1],dp[1][n-1]);
    cout << ans << endl;


    return 0;
}