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

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

const int MAXN = 110;
ll dp[MAXN][2];

int N, D;
ll T[MAXN], K[MAXN];

int solve(int idx, int s) {
    ll& res = dp[idx][s];
    if (~res) return res;
    if (idx >= N) {
        res = 0;
    }
    else {
        if (s == 0) res = max(solve(idx+1, s)+T[idx], solve(idx+1, (s+1)%2)-D+K[idx]);
        else res = max(solve(idx+1, s)+K[idx], solve(idx+1, (s+1)%2)-D+T[idx]);
    }
    return res;
}

int main() {
    cin >> N >> D;
    fill(dp[0], dp[N], -1);
    rep(i, N) cin >> T[i] >> K[i];
    cout << solve(0, 0) << endl;
    return 0;
}