#include <iostream>

using namespace std;
using LL = long long;
using ULL = unsigned long long;

int dp[101][2];
int T[101], K[101];

int main() {
    int N, D;
    cin >> N >> D;

    for (int i = 1; i <= N; i++) cin >> T[i] >> K[i];

    dp[1][0] = T[1];
    dp[1][1] = K[1] - D;
    for (int i = 2; i <= N; i++) {
        dp[i][0] = max(T[i] + dp[i - 1][0], T[i] + dp[i - 1][1] - D);
        dp[i][1] = max(K[i] + dp[i - 1][1], K[i] + dp[i - 1][0] - D);
    }

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