#include <iostream>
#include <algorithm>
using namespace std;

int t[105],k[105];

int main(void){
    int n,d;cin>>n>>d;
    for(int i=0;i<n;i++){
        cin>>t[i]>>k[i];
    }
    int at=t[0],ak=k[0]-d;
    for(int i=1;i<n;i++){
        int aT=at,aK=ak;
        at=max(aT+t[i],aK+t[i]-d);
        ak=max(ak+k[i],aT+k[i]-d);
    }
    cout<<max(at,ak)<<endl;
    
    return 0;
}