#include <iostream>
#include <vector>
using namespace std;

int main(void) {
    int N, D;
    vector<int> T, K;

    cin >> N >> D;

    T.resize(N);
    K.resize(N);

    for (int i = 0; i < N; i++) cin >> T[i] >> K[i];

    // 0: Tokyo
    // 1: Kyoto
    int dp[100][2];

    dp[0][0] = T[0];
    dp[0][1] = K[0] - D;

    for (int i = 1; i < N; i++) {
        dp[i][0] = max(dp[i-1][0] + T[i], dp[i-1][1] + T[i] - D);
        dp[i][1] = max(dp[i-1][0] + K[i] - D, dp[i-1][1] + K[i]);
    }

    cout << max(dp[N-1][0], dp[N-1][1]) << endl;

    return 0;
}