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#include<bits/stdc++.h>
using namespace std;
#define ll long long

const int n_max = 5 * 1e5+7;
ll dp_color[n_max];
ll used[n_max];
ll dp[n_max];

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    ll n, c;
    cin >> n >> c;
    
    int f_id = 0;
    int last = 0;
    for(int i = 1; i <= n; i++){
        int a, w;
        cin >> a >> w;
        dp[i] = dp[i-1];
        if(last != a){
            f_id = i - 1;
            last = a;
        }
        if(used[w] == a)continue;

        used[w] = a;
        ll tmp = max(a + dp[f_id] - c, a + dp_color[w]);
        dp_color[w] = max(dp_color[w], tmp);
        dp[i] = max(dp[i], tmp);
    }
    cout << dp[n] << '\n';



    return 0;
}