#include <bits/stdc++.h>
using namespace std;
typedef  long long int ll;

int n, d;
int t[110], k[110];

ll f(int i, int j) {
    if (i == n)
        return 0;
    
    if (j == 0) {//current tokyo
        if (k[i]-d > t[i]) {
            return f(i+1, 1) + k[i]-d;
        } else {
            return f(i+1, 0) + t[i];
        }
    } else {//current kyoto
        if (t[i]-d > k[i]) {
            return f(i+1, 0) + t[i]-d;
        } else {
            return f(i+1, 1) + k[i];
        }
    }
    return 0;
}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    cin >> n >> d;
    for (int i = 0; i < n; i++) {
        cin >> t[i] >> k[i];
    }
    cout << f(0, 0) << endl;
    
    return 0;
}