C++ :

#include <iostream>
#include <algorithm>

using namespace std;

typedef struct tagSupply
{
	int m_price;
	int m_amount;
} Supply;

int main()
{
	int comp(const Supply& a, const Supply& b);
	int n = 0, m = 0;
	int nSum = 0;
	Supply* p = NULL;

	cin >>n >>m;
	p = new Supply[m];
	for (int i = 0; i < m; ++i)
	{
		cin >>p[i].m_price >>p[i].m_amount;
	}
	sort(p, p+m, comp);
	
	int i = 0;
	while (n > 0)
	{
		if (n >= p[i].m_amount)
		{
			nSum += p[i].m_amount * p[i].m_price;
			n -= p[i].m_amount;
		}
		else
		{
			nSum += n * p[i].m_price;
			n = 0;
		}
		++i;
	}
	
	cout <<nSum;
	
	delete []p;
	return 0;
}

int comp(const Supply& a, const Supply& b)
{
	return a.m_price < b.m_price;
}