#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MOD = 1000000007;
const int MAX_N = 110;
const int INF = 1<<29;

#define REP(i,n) for((i)=0;(i)<(int)(n);(i)++)

int dp[MAX_N][2];
int c[MAX_N][2];

int main(){
    int n, D;
    cin >> n >> D;
    for (int i = 0; i < n; i++){
        cin >> c[i][0] >> c[i][1];
    }

    dp[0][0] = c[0][0];
    dp[0][1] = c[0][1] - D;
    for (int i = 0; i < n; i++){
        dp[i + 1][0] = c[i + 1][0];
        dp[i + 1][0] += max(dp[i][0], dp[i][1] - D);
        dp[i + 1][1] = c[i + 1][1];
        dp[i + 1][1] += max(dp[i][1], dp[i][0] - D);
    }

    cout << max(dp[n][0], dp[n][1]) << endl;

    return 0;
}