Yogurt factory

#include <iostream>
#include <algorithm>
using namespace std;
#define maxn 10005
typedef long long ll;
ll c[maxn], y[maxn];
int main()
{
    ll cost = 0;
    ll n, s;
    cin >> n >> s;
    for (ll i = 0; i < n; ++i)
        cin >> c[i] >> y[i];
    for (ll i = 1; i < n;++i)
        c[i] = min(c[i], c[i - 1] + s);
    for (ll i = 0; i < n; ++i)
        cost += c[i] * y[i];
    cout << cost << endl;
    return 0;
}