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

#define REP(i, m, n) for (int i = (m); i < (int)(n); i++)
#define REPS(i, m, n) for (int i = (m); i <= (int)(n); i++)
#define rep(i, n) REP(i, 0, n)
#define reps(i, n) for (int i = 0; i <= (int)(n); i++)
#define rrep(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define rreps(i, x) for (int i = (int)(x); i >= 0; i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pb push_back
typedef long long ll;
typedef pair<int, int> P;
const int inf = INT_MAX;
const ll INF = 1LL << 60;
const ll mod = 1e9 + 7;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val) { fill( (T*)array, (T*)(array+N), val ); }

int dp[101][2], C[101][2];
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N, D; cin >> N >> D;
    rep(i, N) cin >> C[i][0] >> C[i][1];
    Fill(dp, -inf);
    dp[0][0] = 0;
    dp[0][1] = -1;
    rep(d, N) {
        rep(p, 2) {
            dp[d+1][p] = max(dp[d+1][p], dp[d][p] + C[d][p]);
            dp[d+1][1-p] = max(dp[d+1][1-p], dp[d][p] + C[d][1-p] - D);
        }
    }
    cout << max(dp[N][0], dp[N][1]) << endl;
    return 0;
}