/*
 * FileName:     no_561_tokyo_kyoto
 * CreatedDate:  2020-04-04 10:48:11 +0900
 * LastModified: 2020-04-04 11:18:42 +0900
 */

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