#include "bits/stdc++.h"
using namespace std;
#define int long long
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define RFOR(i, a, b) for(int i=(b-1);i>=(a);i--)
#define REP(i, n) for(int i=0; i<(n); i++)
#define RREP(i, n) for(int i=(n-1); i>=0; i--)
#define ALL(a) (a).begin(),(a).end()
#define UNIQUE_SORT(l) sort(ALL(l)); l.erase(unique(ALL(l)), l.end());
#define CONTAIN(a, b) find(ALL(a), (b)) != (a).end()
#define array2(type, x, y) array<array<type, y>, x>
#define vector2(type) vector<vector<type> >
#define out(...) printf(__VA_ARGS__)

int dxy[] = {0, 1, 0, -1, 0};

void solve();
signed main()
{
#if DEBUG
    std::ifstream in("input.txt");
    std::cin.rdbuf(in.rdbuf());
#endif
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
    return 0;
}

/*================================*/

int N,D;
int T[112],K[112];
// i日目, Tokyo or Kyotoに滞在 = 利益
int DP[112][2];

void solve() {
    cin>>N>>D;
    REP(i,N) cin>>T[i]>>K[i];
    
    DP[0][0] = T[0];
    DP[0][1] = K[0]-D;
    FOR(i,1,N) {
        DP[i][0] = max(DP[i-1][0], DP[i-1][1]-D) + T[i];
        DP[i][1] = max(DP[i-1][1], DP[i-1][0]-D) + K[i];
    }
    cout << max(DP[N-1][0], DP[N-1][1]) << endl;
}