#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

const ll INF = 1e9;
int main() {
    ll N, D;
    cin >> N >> D;
    ll T[N], K[N];
    int i, j, k;
    for(i = 0; i < N; i++) {
        cin >> T[i] >> K[i];
    }
    ll dp[2][2];
    dp[0][0] = 0;
    dp[0][1] = -INF;
    dp[1][0] = -INF;
    dp[1][1] = -INF;
    for(i = 0; i < N;i++){
        for(j = 0; j < 2;j++){
            dp[1][0] = max(dp[0][0] + T[i], dp[0][1] + T[i] - D);
            dp[1][1] = max(dp[0][0] + K[i] - D, dp[0][1] + K[i]);
        }
        dp[0][0] = dp[1][0];
        dp[0][1] = dp[1][1];
    }
    cout << max(dp[1][0], dp[1][1]) << endl;
    return 0;
}