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#include<bits/stdc++.h>

using namespace std;

const int N = 500007;

int t[N];

int max(int a, int b) {
    if (a > b) return a;
    return b;
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, c, a, b, mw, mc, la;
    cin>>n>>c;
    cin>>a>>b;
    mw = 0; mc = 0; la = a;
    t[b] = a;
    stack<pair<int, int> > S;
    S.push({b, a});
    for (int i = 1; i < n; ++i) {
        cin>>a>>b;
        if (la < a) {
            while (!S.empty()) {
                int x = S.top().second;
                int k = S.top().first;
                // cout<<"### "<<x<<' '<<k<<'\n';
                if (x > mw) {
                    mw = x;
                    mc = k;
                }
                t[k] = x;
                S.pop();
            }
        }
        int w = max(a + t[b], mw + a - (mc != b) * c);
        S.push({b, w});
        la = a;
    }
    while (!S.empty()) {
        int x = S.top().second;
        int k = S.top().first;
        // cout<<"### "<<x<<' '<<k<<'\n';
        if (x > mw) {
            mw = x;
            mc = k;
        }
        t[k] = x;
        S.pop();
    }
    cout<<mw<<'\n';
}