#include <cstdio>
#include <vector>

using namespace std;

vector<int> valueForTime[1505];
int main() {
    freopen("carnati.in","r",stdin);
    freopen("carnati.out","w",stdout);
    int n,c;
    scanf("%d %d",&n,&c);
    for(int i=1;i<=n;++i) {
        int passingLove, value;
        scanf("%d %d",&passingLove,&value);
        valueForTime[passingLove].emplace_back(value);
    }
    int ans=-1;
    for(int i=0;i<=1500;++i) {
        for(int j=0;j<valueForTime[i].size();++j) {
            int currMax=0, maxx=0;
            for(int k=0;k<=1500;++k) {
                currMax-=c;
                for(int g=0;g<valueForTime[k].size();++g) {
                    if(valueForTime[k][g]>=valueForTime[i][j]) {
                        currMax+=valueForTime[i][j];
                    }
                }
                maxx= max(maxx, currMax);
                currMax= max(currMax, 0);
            }
            ans=max(ans, maxx);
        }
    }
    printf("%d\n", ans);
    return 0;
}