#include <bits/stdc++.h>
#define REP(i, s, n) for (int i = s; i < n; i++)
#define ALL(a) a.begin(), a.end()
#define MOD 1000000007
using namespace std;
typedef long long ll;

int main() {
    int N, D; cin >> N >> D;
    vector<vector<int>> TK(N, vector<int>(2));
    REP(i, 0, N) cin >> TK[i][0] >> TK[i][1];

    vector<vector<int>> dp(N + 1, vector<int>(2, -(1 << 30)));
    dp[0][0] = 0;
    REP(i, 0, N) {
        REP(j, 0, 2) {
            REP(k, 0, 2) {
                dp[i + 1][k] = max(dp[i + 1][k], dp[i][j] + TK[i][k] - ((j == k) ? 0 : D));
            }
        }
    }
    cout << max(dp[N][0], dp[N][1]) << endl;
    return 0;
}