#include <iostream>
using namespace std;
int main(void){
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int n,d;
    cin >> n >> d;
    int dp[2][n+1];
    int t1,k1;
    cin >> t1 >> k1;
    dp[0][1] = t1;
    dp[1][1] = k1-d;
    for(int i = 2; i <=n;i++){
        int t,k;
        cin >> t >> k;
        dp[0][i]=max(dp[0][i-1]+t,dp[1][i-1]+t-d);
        dp[1][i]=max(dp[0][i-1]+k-d,dp[1][i-1]+k);
    }
    cout << max(dp[0][n],dp[1][n]) << endl;
}