B. Burglar and Matches

B. Burglar and Matches-codeforces

// working.cpp by Bill Weinman <http://bw.org/>
#include <bits/stdc++.h>
using namespace std;
int main() {
	int n,m;
	unsigned long long count1=0;
	unsigned long long count2=0;
	cin>>n>>m;
	vector< vector<int> >container(m,vector<int>(2));
	for(int i=0;i<m;i++)
	{
		for(int j=1;j>=0;j--)
			cin>>container[i][j];
	}
	sort(container.begin(),container.end());
	for(int i=m-1;i>=0;i--)
		{
				if(count2+container[i][1]<n)
					count2+=container[i][1],count1+=container[i][0]*container[i][1];
				else
				{
					count1+=(n-count2)*container[i][0];
					break;
				}
		}
	cout<<count1<<endl;
    return 0;
}