#include <iostream>

#define MAXN 10010
#define INF 9999999

using namespace std;

long long n,s,ans,c[MAXN],y[MAXN];
//c means cost
//y means the requares

void run()
{
	long long best=INF;
	ans=0;
	for (int i=1;i<=n;i++)
	{
		best+=s;
		if (c[i]<best)
			best=c[i];
		ans+=best*y[i];
	}
}

void ini()
{
	cin>>n>>s;
	for (int i=1;i<=n;i++)
	{
		cin>>c[i]>>y[i];
	}
}

int main()
{
	freopen("factory.in","r",stdin);
	freopen("factory.out","w",stdout);
	ini();
	run();
	cout<<ans;
	return 0;
}