#include<bits/stdc++.h>
using namespace std;
#define rep(i,N) for(int (i)=0;(i) < (N); (i)++)

int main(){
    int N,D;
    cin >> N >> D;
    vector<long> T(N),K(N);
    rep(i,N) cin >> T[i] >> K[i];
    
    vector<vector<long>> dp(N,vector<long>(2));

    #define tokyo 0
    #define kyoto 1
    dp[0][tokyo] = T[0];
    dp[0][kyoto] = K[0]-D;

    for(int i=1;i<N;i++){
        dp[i][tokyo] = max(dp[i-1][tokyo]+T[i],dp[i-1][kyoto]+T[i]-D);
        dp[i][kyoto] = max(dp[i-1][tokyo]+K[i]-D, dp[i-1][kyoto]+K[i]);
    }

    cout << max(dp[N-1][tokyo],dp[N-1][kyoto]) << endl;
}