#include <bits/stdc++.h>
using namespace std;

#define DUMP(x) cerr << #x << "=" << x << endl
#define DUMP2(x, y) cerr<<"("<<#x<<", "<<#y<<") = ("<<x<<", "<<y<<")"<< endl
#define BINARY(x) static_cast<bitset<16> >(x)

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define REP(i,m,n) for (int i=m;i<(int)(n);i++)

#define in_range(x, y, w, h) (0<=(int)(x) && (int)(x)<(int)(w) && 0<=(int)(y) && (int)(y)<(int)(h))
#define ALL(a) (a).begin(),(a).end()

typedef long long ll;
const int INF   = 1e9;
const ll  INFLL = 1e18;
typedef pair<int, int> PII;
int dx[4]={0, -1, 1, 0}, dy[4]={-1, 0, 0, 1};

const int MAX_N = 101;
ll dp[MAX_N][2];

int main()
{
    ios::sync_with_stdio(false);

    int N, D;
    cin >> N >> D;

    rep(i, N) {
        vector<int> A(2);
        rep(_, 2) cin >> A[_];

        rep(j, 2) {
            rep(k , 2) {
                if (i == 0 && k == 1) continue;
                dp[i+1][j] = max(dp[i+1][j], dp[i][k] + A[j] - (j == k ? 0 : D));
            }
        }
    }

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