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#include <cstdio>
#include <cstdint>
#include <vector>
#include <bitset>
#include <tuple>
#include <algorithm>
#include <climits>
#include <numeric>
#include <iostream>
#include <cstring>
#include <iomanip>
#include <cmath>
#include <map>
#include <set>
#include <queue>

using namespace std;

map<int, long long> bestHeightPerColor;
map<long long, set<int>> colorsAchievableFromHeight;
map<int, set<long long>> heightsAchievebleFromColor;
priority_queue<long long> topHeights;

int n, c;

map<int, set<int>> inputData;//height -> list of colors
vector<pair<int,int>> inputDataFlat; //height, color

long long findBestHeightOfOtherColor(int color)
{
    queue<long long> comeBacks;
    long long result = -1;
    while(!topHeights.empty() && result == -1)
    {
        long long topHeightOtherColor = topHeights.top();
        int possibilitiesToAchieve = colorsAchievableFromHeight[topHeightOtherColor].size();
        topHeights.pop();
        if(possibilitiesToAchieve == 0)
        {
            continue;
        }
        if(colorsAchievableFromHeight[topHeightOtherColor].find(color) != colorsAchievableFromHeight[topHeightOtherColor].end())
        {
            if (possibilitiesToAchieve > 1)
            {
                result = topHeightOtherColor;
            }
        }
        else
        {
            result = topHeightOtherColor;
        }
        comeBacks.push(topHeightOtherColor);
    }
    while (!comeBacks.empty())
    {
        long long comeBack = comeBacks.front();
        topHeights.push(comeBack);
        comeBacks.pop();
    }
    return result;
}

long long findUpdatedHeightForColor(int color, long long additionalHeight)
{
    long long result = bestHeightPerColor[color] + additionalHeight;
    long long newCandidate = additionalHeight + findBestHeightOfOtherColor(color) - c;

    //cout<<"condidering :"<<additionalHeight<<" "<<color<<endl;
    //cout<<"best continuation: "<<result<<endl;
    //cout<<"best z kara: "<<newCandidate<<endl;

    result = max(result, newCandidate);
    return result;
}

void updateHeightForColor(int color, long long newHeight)
{
    for(auto itColor = heightsAchievebleFromColor[color].begin(); itColor!=heightsAchievebleFromColor[color].end(); ++itColor)
    {
        long long achievableHeight = (*itColor);
        colorsAchievableFromHeight[achievableHeight].erase(color);
    }
    heightsAchievebleFromColor[color].clear();

    bestHeightPerColor[color] = newHeight;
    heightsAchievebleFromColor[color].insert(newHeight);
    colorsAchievableFromHeight[newHeight].insert(color);

    topHeights.push(newHeight);
}




void init()
{
    scanf("%d %d", &n, &c);

    int lastA = -1;

    topHeights.push(0);
    for (int i = 0; i < n; i++)
    {
        int a, w;
        scanf("%d %d", &a, &w);
        inputData[a].insert(w);
        bestHeightPerColor[w] = 0;
        heightsAchievebleFromColor[w].insert(0);
        colorsAchievableFromHeight[0].insert(w);
    }

    for (auto itA = inputData.begin(); itA!=inputData.end(); ++itA ) {
        int inputHeight = itA->first;

        map <int, long long>colorToNewValue;

        for(auto itW = itA->second.begin(); itW!= itA->second.end(); ++itW)
        {
            int inputColor = *(itW);

            long long newValue = findUpdatedHeightForColor(inputColor, inputHeight);
            colorToNewValue[inputColor] = newValue;
        }
        for(auto it = colorToNewValue.begin(); it != colorToNewValue.end(); ++it)
        {
            updateHeightForColor(it->first, it->second);
        }
    }
}

int main()
{
    init();
    long long result = topHeights.top();

    printf("%lld\n", result);
}