#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <iomanip>
#include <random>
#include <cassert>
#include <cstring>
using namespace std;

#define GET_ARG(a,b,c,F,...) F
#define REP3(i,s,e) for (i = s; i <= e; i++)
#define REP2(i,n) REP3 (i,0,(int)(n)-1)
#define REP(...) GET_ARG (__VA_ARGS__,REP3,REP2) (__VA_ARGS__)
#define RREP3(i,s,e) for (i = s; i >= e; i--)
#define RREP2(i,n) RREP3 (i,(int)(n)-1,0)
#define RREP(...) GET_ARG (__VA_ARGS__,RREP3,RREP2) (__VA_ARGS__)
#define DEBUG(x) cerr << #x ": " << x << endl

const int INF = 1e9;

int dp[2][101];

int main(void) {
    int i, n, d;
    cin >> n >> d;
    REP (i,0,n) dp[0][i] = dp[1][i] = -INF;
    dp[0][0] = 0;
    REP (i,n) {
        int t, k;
        cin >> t >> k;
        dp[0][i+1] = max(dp[0][i] + t, dp[1][i] - d + t);
        dp[1][i+1] = max(dp[1][i] + k, dp[0][i] - d + k);
    }
    cout << max(dp[0][n],dp[1][n]) << endl;
    return 0;
}