#include <bits/stdc++.h>

using namespace std;

const int MAX = 111;
int n, d;
long long t[MAX], k[MAX];
unordered_set<long long> dp[2];

int main() {
    cin >> n >> d;
    for (int i = 0; i < n; i++) {
        cin >> t[i] >> k[i];
    }
    dp[0].insert(0);
    for (int i = 0; i < n; i++) {
        unordered_set<long long> ndp[2];
        for (long long e : dp[0]) {
            ndp[0].insert(e + t[i]);
            ndp[1].insert(e + k[i] - d);
        }
        for (long long e : dp[1]) {
            ndp[1].insert(e + k[i]);
            ndp[0].insert(e + t[i] - d);
        }
        for (int j = 0; j < 2; j++) {
            dp[j] = ndp[j];
        }
    }
    long long ans = 0;
    for (int i = 0; i < 2; i++) {
        for (long long e : dp[i]) ans = max(ans, e);
    }
    cout << ans << endl;
    return 0;
}