#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];
int dp[101][2];

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

    cin >> n >> d;
    rep(i, n) cin >> t[i] >> k[i];
    
    dp[0][0] = dp[0][1] = 0;
    dp[1][0] = t[0];
    dp[1][1] = k[0] - d;
    FOR(i, 1, n) {
        dp[i + 1][0] = max(dp[i][0] + t[i], dp[i][1] + t[i] - d);
        dp[i + 1][1] = max(dp[i][0] + k[i] - d, dp[i][1] + k[i]);
    }
    cout << max(dp[n][0], dp[n][1]) << endl;
    return 0;
}