/*
 * FileName:     no_561_tokyo_kyoto
 * CreatedDate:  2020-04-04 10:48:11 +0900
 * LastModified: 2020-04-04 20:36:04 +0900
 */

#include <stdio.h>
long int max(long int s, long int t){
    if(s>t){ return s; }
    else{ return t; }
}
long int dp( int i, int N, long int D, long int T[], long int K[], int bi){
    if(i==N){
        return 0;
    }
    else{
        long int box_0,box_1;
        if( bi==0 ){
            box_0 = dp(i+1,N,D,T,K,0)+T[i];
            box_1 = dp(i+1,N,D,T,K,1)+K[i]-D;
            return max(box_0,box_1);
        }
        else if( bi==1 ){
            box_0 = dp(i+1,N,D,T,K,0)+T[i]-D;
            box_1 = dp(i+1,N,D,T,K,1)+K[i];
            return max(box_0,box_1);
        }
    }
}
int main(void){
    int N;
    long int D;
    scanf("%d %ld",&N,&D);
    long int T[N],K[N];
    for(int i=0;i<N;i++){
        scanf("%ld %ld",&T[i], &K[i]);
    }
    long int tmp_0 = dp(0,N,D,T,K,0);
    long int tmp_1 = dp(0,N,D,T,K,1)-D;
    
    printf("%ld\n",max(tmp_0,tmp_1));
    return 0;
}