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#include <iostream>

using namespace std;
typedef long long ll;

const int N = 5e5+9;
int A[N], W[N], last[N];
ll MX_W[N], dp[N];

int main() {

    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n, c; cin >> n >> c;
    ll res = 0, dp_mx = -1, prev_a = -1, mx_lower = -1;
    for(int i = 1; i <= n; ++i) {
        int a, w; cin >> a >> w;
        A[i] = a;
        W[i] = w;
        ll mx = -1;
        if(last[w]) {
            if(A[last[w]] < a) {
                mx = dp[last[w]];
            }
            else {
                mx = MX_W[last[w]];
            }
        }
        MX_W[i] = mx;
        dp[i] = a;
        if(mx != -1) {
            dp[i] = max(dp[i], mx+a);
        }
        mx = -1;
        if(prev_a < a) {
            mx_lower = dp_mx;
        }
        mx = mx_lower;
        if(mx != -1) {
            dp[i] = max(dp[i], mx+a-c);
        }
        res = max(res, dp[i]);

        last[w] = i;
        dp_mx = max(dp_mx, dp[i]);
        prev_a = a;
    }
    cout << res << "\n";

    return 0;
}