/*
 * FileName:     no_561_tokyo_kyoto
 * CreatedDate:  2020-04-04 10:48:11 +0900
 * LastModified: 2020-04-04 11:10:50 +0900
 */

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