#include <bits/stdc++.h>
#define int long long

using namespace std;
using LL = long long;
using P = pair<int, int>;
using Tapris = tuple<int, int, int>;

#define FOR(i, a, n) for(int i = (int)(a); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)

#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

const int INF = (int)1e9;
const LL INFL = (LL)1e15;
const int MOD = 1e9 + 7;

int dy[]={0, 0, 1, -1, 0};
int dx[]={1, -1, 0, 0, 0};

/*************** using variables ***************/
int n, d;
vector<int> t, k;
int dp[105][2];
/**********************************************/

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

    cin >> n >> d;
    t.assign(n, 0);
    k.assign(n, 0);
    REP(i, n) cin >> t[i] >> k[i];

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

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

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