#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
using i32 = int_fast32_t;
using i64 = int_fast64_t;
#define rep(i, n) for (i32 i = 0; i < (i32)(n); i++)
#define all(a) (a).begin(),(a).end()
#define rall(a) (a).rbegin(),(a).rend()
using P = pair<i64,i64>;

int main(){
ios::sync_with_stdio(false);
std::cin.tie(nullptr);
i64 n,d;
cin >> n >> d;
vector<i64> t(n),k(n);
rep(i,n)cin >> t[i] >> k[i];
vector<vector<i64>> dp(n + 1,vector<i64>(2,-1));
dp[0][0] = 0;
dp[0][1] = 0;
rep(i,n){
    rep(j,2){
        rep(l,2){
           if(j == l){
               if(l == 0){
                   dp[i + 1][l] = max(dp[i][l] + t[i],dp[i + 1][l]);
               }else{
                   if(i == 0){
                        dp[i + 1][l] = max(dp[i][l] + k[i] - d,dp[i + 1][l]);
                   }else{
                       dp[i + 1][l] = max(dp[i][l] + k[i],dp[i + 1][l]);
                   }
               }
           }else{
               if(l == 0){
                   dp[i + 1][l] = max(dp[i][j] + t[i] - d,dp[i + 1][l]);
               }else{
                   dp[i + 1][l] = max(dp[i][j] + k[i] - d,dp[i + 1][l]);
               }
           }
        }
    }
}
/*rep(i,n + 1){
    rep(j,2){
        cout << dp[i][j] << " \n"[j == 1];
    }
}*/
cout << max(dp[n][0],dp[n][1]) << endl;
}