#include <iostream>
#include <algorithm>
#include <cstring>

using namespace std;

int const INF = -1;

int N, D, T, K, T1, K1;
int dp[110][2];

int main()
{
    cin >> N >> D;
    
    memset(dp, INF, sizeof(dp));

    cin >> T1 >> K1;
    dp[0][0] = T1;
    dp[0][1] = K1 - D;
    
    for(int i = 0; i < N - 1; i++)
    {
        cin >> T >> K;
        dp[i + 1][0] = max(dp[i][0] + T, dp[i][1] + T - D);
        dp[i + 1][1] = max(dp[i][1] + K, dp[i][0] + K - D);
    }

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