#include "bits/stdc++.h"
using namespace std;

using ll = long long;
#define mset(a,x) memset(a,x,sizeof(a))

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

ll dp[110][2];
ll rec(int i, int d) {
    ll &r = dp[i][d];
    if (r != -1) return r;
    if (i == N) return r = 0;
    if (d == 0) // tokyo
        return r = max(rec(i + 1, d) + T[i], rec(i + 1, ~d) + K[i] - D);
    else
        return r = max(rec(i + 1, d) + K[i], rec(i + 1, ~d) + T[i] - D);
}

int main() {
    cin >> N >> D;
    for (int i = 0; i < N; i++) scanf("%d%d", T + i, K + i);
    mset(dp, -1);
    cout << rec(0, 0) << endl;
    return 0;
}