#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];

ll dp[MAX_N][2];

// i 日目まで働いて今自分が cur にいるときの最大値
ll dfs(ll i, ll cur) {
    if(i >= N) { return 0; }
    ll ans;
    if(cur == 0) {
        ans = max(dfs(i+1, 0) + T[i], dfs(i+1, 1) - D + K[i]);
    } else {
        ans = max(dfs(i+1, 1) + K[i], dfs(i+1, 0) - D + T[i]);
    }
    return ans;
}

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

    ZERO(dp);
    for(ll i = N - 1; i >= 0; --i) {
        for(int j = 0; j < 2; ++j) {
            if(j == 0) {
                dp[i][j] = max(dp[i+1][0] + T[i], dp[i+1][1] - D + K[i]);
            } else {
                dp[i][j] = max(dp[i+1][1] + K[i], dp[i+1][0] - D + T[i]);
            }
        }
    }
    //ll ans = dfs(0, 0);
    //printf("%lld\n", ans);
    printf("%lld\n", dp[0][0]);
    return 0;
}