#include <bits/stdc++.h>  

using namespace std;

//#define int long long
#define FOR(i, j, k) for(int i = j; i < k; ++i)
#define rep(i, j) FOR(i, 0, j)
#define INF (1 << 30)

typedef unsigned long long ull;
typedef pair<int, int> P;
typedef pair<P, int> Pi;

const int MOD = 1e9 + 7;
const int dy[] = {0, 0, 1, -1};
const int dx[] = {1, -1, 0, 0};

template <class T> void chmin(T& a, const T& b) { a = min(a, b); }
template <class T> void chmax(T& a, const T& b) { a = max(a, b); }

int n, d, ans = 0;
int t[100], k[100];

void solve(int res, int cnt, int now) {
    if(cnt == n) {
        chmax(ans, res);
        return;
    }
    if(now == 0 || cnt == 0) {
        solve(res + t[cnt], cnt + 1, 0);
        solve(res + k[cnt] - d, cnt + 1, 1);
    }
    else {
        solve(res + t[cnt] - d, cnt + 1, 0);
        solve(res + k[cnt], cnt + 1, 1);
    }
}

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

    cin >> n >> d;
    rep(i, n) cin >> t[i] >> k[i];
    solve(0, 0, 0);
    cout << ans << endl;
    return 0;
}