#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

#define REP(i,n) for(ll i=0;i<n;++i)
#define SORT(name) sort(name.begin(), name.end())
#define ZERO(p) memset(p, 0, sizeof(p))
#define MINUS(p) memset(p, -1, sizeof(p))

#define MOD 1000000007
#define INF 1000000000
#define MAX_N 110

ll N, D;
ll T[MAX_N];
ll K[MAX_N];

// i 日目まで働いて今自分が cur にいて、その時の手持ちが cost のときの最大値
ll dfs(ll i, ll cur, ll cost) {
    if(i >= N) { return cost; }
    ll ans;
    if(cur == 0) {
        ans = max(dfs(i+1, 0, cost + T[i]), dfs(i+1, 1, cost - D + K[i]));
    } else {
        ans = max(dfs(i+1, 1, cost + K[i]), dfs(i+1, 0, cost - D + T[i]));
    }
    return ans;
}

int main()
{
    cin >> N >> D;
    REP(i, N) {
        cin >> T[i] >> K[i];
    }
    ll ans = dfs(0, 0, 0);
    printf("%lld\n", ans);
    return 0;
}