#include<bits/stdc++.h>
typedef long long ll;
using namespace std;
int INF = 1e9;
int MOD = 1e9+7;
int dp[2][1000];
main(){
    int N,D,T[1000],K[1000];
    cin >> N >> D;
    for(int i = 0;i < N;i++)cin >> T[i] >> K[i];
    dp[0][0] = T[0];
    dp[1][0] = K[0] - D;
    for(int i = 1;i < N;i++){
        dp[0][i] = max(dp[0][i-1]+T[i]  ,dp[1][i-1]+T[i]-D);
        dp[1][i] = max(dp[0][i-1]+K[i]-D,dp[1][i-1]+K[i]);
    }
    cout << max(dp[0][N-1],dp[1][N-1]) << endl;
}