#include <bits/stdc++.h>
#define FOR(i,bg,ed) for(ll i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
#define int long long
using namespace std;
typedef long long ll;
typedef vector<vector<ll>> mat;
const int INF = 1e9;

int N, D;
int T[110], K[110];

struct edge { int from, to, cost; };
edge es[2010];
int d[1000];

int V, E;

//s番目の頂点から各頂点への最短距離を求める
void shortest_path(int s)
{
    REP(i,V) d[i] = 1e14;
    d[s] = 0;
    while (true) {
        bool update = false;
        REP(i,E) {
            edge e = es[i];
            if (d[e.from] != 1e14 && d[e.to] > d[e.from] + e.cost) {
                d[e.to] = d[e.from] + e.cost;
                update = true;
            }
        }
        if (!update) break;
    }
}

signed main()
{
    cin >> N >> D;
    REP(i,N) cin >> T[i] >> K[i];

    V = 4 * N + 2;

    E = 0;
    es[E++] = (edge){0, 1, 0};
    for (int i=1; i<=2*N; i+=2) {
        es[E++] = (edge){i, i + 1, -T[i / 2]};
        es[E++] = (edge){i + 2 * N + 1, i + 2 * N + 2, -K[i / 2]};
    }

    for (int i=0; i<N; i++) {
        es[E++] = (edge){i * 2, i * 2 + 2 * N + 2, D};
        es[E++] = (edge){i * 2, i * 2 + 1, 0};
        es[E++] = (edge){i * 2 + 2 * N + 1, i * 2 + 1, D};
        es[E++] = (edge){i * 2 + 2 * N + 1, i * 2 + 2 * N + 2, 0};
    }

    shortest_path(0);

    /*
    REP(i,4*N+2) {
        cout << "i = " << i << " : d[i] = " << d[i] << endl;
    }
    */

    cout << max(-d[2 * N], -d[4 * N + 1]) << endl;
}