#include<stdio.h>
#include<algorithm>
typedef struct data
{
	int value;
	int weight;
}data;
bool acompare(data lhs, data rhs)
{
    return (double)lhs.value/lhs.weight > (double)rhs.value/rhs.weight;
}

	data arr[100];
int main()
{
	int n,i;
		int k,tp=0;
	scanf("%d",&n);

	for(i=0;i<n;i++)
	{
		 scanf("%d", &arr[i].weight);
        scanf("%d", &arr[i].value);
	}
	 sort(arr, arr+n, acompare);

	scanf("%d",&k);

	for(i=0;i<n;i++)
	{
		while(k!=0)
		{
			if(arr[i].weight<=k)
			{
				tp+=arr[i].value;
				k-=arr[i].weight;
			}
			else
			{
				tp+=(arr[i].value/arr[i].weight)*k;
				break;
			}
		}
	}
	printf("%d\n",tp);

}