#include <algorithm> #include <cassert> #include <cctype> #include <cmath> #include <cstdio> #include <cstdlib> #include <ctime> #include <deque> #include <functional> #include <iomanip> #include <iostream> #include <list> #include <map> #include <queue> #include <random> #include <set> #include <sstream> #include <string> #include <utility> #include <vector> #define REP(i,s,n) for(int i=(int)(s);i<(int)(n);i++) using namespace std; typedef long long int ll; typedef vector<int> VI; typedef vector<ll> VL; typedef pair<int, int> PI; const ll mod = 1e9 + 7; const ll inf = 5e15; // 5000chou int main(void) { int n; ll d; cin >> n >> d; VL t(n), k(n); REP(i, 0, n) { cin >> t[i] >> k[i]; } vector<VL> dp(n + 1, VL(2, -inf)); // 0: tokyo , 1: kyoto dp[0][0] = 0; REP(i, 0, n) { dp[i + 1][0] = max(dp[i][0], dp[i][1] - d) + t[i]; dp[i + 1][1] = max(dp[i][1], dp[i][0] - d) + k[i]; } cout << max(dp[n][0], dp[n][1]) << endl; }