#include <bits/stdc++.h>
using namespace std;

int main(){
    int N, D;
    cin >> N >> D;
    vector<int> T(N),K(N);
    for(int i = 0; i < N; i++) cin >> T.at(i) >> K.at(i);
    vector<vector<long long>> dp(N+1, vector<long long>(2,-(1LL << 60)));
    dp.at(0).at(0) = 0;
    for(int i = 0; i < N; i++){
        dp.at(i+1).at(0) = max(dp.at(i).at(1) - D + T.at(i), dp.at(i).at(0) + T.at(i));
        dp.at(i+1).at(1) = max(dp.at(i).at(0) - D + K.at(i), dp.at(i).at(1) + K.at(i));
    }
    cout << max(dp.at(N).at(0), dp.at(N).at(1)) << endl;
}