#include <bits/stdc++.h>
using namespace std;
const int IINF = INT_MAX;
#define REP(i, n) for (int i = 0; i < n; i++)

int main() {
    int n, d, dp[2][105]{};
    cin >> n >> d;
    dp[1][0] = -IINF/2;
    REP(i,n){
        int t, k;
        cin >> t >> k;
        dp[0][i+1] = max(dp[0][i]+t, dp[1][i]+t-d);
        dp[1][i+1] = max(dp[1][i]+k, dp[0][i]+k-d);
    }
    cout << max(dp[0][n], dp[1][n]) << endl;
    return 0;
}