#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=1e5+10;
LL c[N], y[N];
int main()
{
    LL n, s; scanf("%lld%lld", &n, &s);
    for(int i=1; i<=n; i++) scanf("%lld%lld", &c[i], &y[i]);
    LL cmin=1e18, ans=0;
    for(int i=1; i<=n; i++)
    {
        cmin=min(cmin + s, c[i]);
        ans += cmin * y[i];
    }
    printf("%lld\n", ans);
    return 0;
}