#include "bits/stdc++.h"
using namespace std;
#define ll long long int
#define rep(i,n) for( int i = 0; i < n; i++ )
#define rrep(i,n) for( int i = n; i >= 0; i-- )
#define REP(i,s,t) for( int i = s; i <= t; i++ )
#define RREP(i,s,t) for( int i = s; i >= t; i-- )
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define INF 2000000000
#define mod 1000000007
#define INF2 1000000000000000000

int dp[102][102];

int main(void)
{
    cin.tie(0);
    ios::sync_with_stdio(false);
	int N, D; cin >> N >> D;
	int T[100][2];
	rep(i, N) cin >> T[i][0] >> T[i][1];
	rep(i, N + 1) dp[i][0] = -i*D;
	rep(i, N) dp[0][i + 1] = dp[0][i] + T[i][0];
	rep(i, N) {
		//rep(j, N) {
		//	dp[i][j + 1] = max(dp[0][j + 1], dp[i][j] + T[j][i % 2]);
		//}
		rep(j, N) {
			dp[i + 1][j + 1] =
				max(dp[i + 1][j] + T[j][(i + 1) % 2],
					dp[i][j] + T[j][(i + 1) % 2] - D);
		}
	}
	//rep(i, N + 1) {
	//	rep(j, N + 1) {
	//		cout << i << " " << j << " " << dp[i][j] << endl;
	//	}
	//}
	int ans = 0;
	rep(i, N + 1) ans = max(ans, dp[i][N]);
	cout << ans << endl;

    return 0;
}