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

int t[200], k[200];
int dp[200][2];

int main(void) {
    int n,d;
    scanf("%d%d",&n,&d);
    for(int i=0; i<n; i++)
        scanf("%d%d",&t[i],&k[i]);

    dp[0][0] = 0;
    dp[0][1] = -d;
    for(int i=0; i<n; i++) {
        dp[i+1][0] = max(dp[i][0], dp[i][1]-d) + t[i];
        dp[i+1][1] = max(dp[i][1], dp[i][0]-d) + k[i];
    }
    printf("%d\n", max(dp[n][0], dp[n][1]));
}