cf 16B

B. Burglar and Matches

time limit per test
0.5 second

memory limit per test
64 megabytes

input
standard input

output
standard output

A burglar got into a matches warehouse and wants to steal as many matches as possible. In the warehouse there are m containers, in thei-th container there are ai matchboxes, and each matchbox contains bi matches. All the matchboxes are of the same size. The burglar's rucksack can hold n matchboxes exactly. Your task is to find out the maximum amount of matches that a burglar can carry away. He has no time to rearrange matches in the matchboxes, that's why he just chooses not more than n matchboxes so that the total amount of matches in them is maximal.

Input
The first line of the input contains integer n (1 ≤ n ≤ 2·108) and integer m (1 ≤ m ≤ 20). The i + 1-th line contains a pair of numbers ai andbi (1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10). All the input numbers are integer.

Output
Output the only number — answer to the problem.

Sample test(s)

input

7 3
5 10
2 5
3 6

output

62

input

3 3
1 3
2 2
3 1

output

7

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<cstdlib>
#include<algorithm>
using namespace std;
int n,m;
struct node
{
int x,y;
}e[30];
bool cmp(node a,node b)
{
return a.y>b.y;
}
int main()
{
int ans=0;
scanf("%d%d",&n,&m);
for(int i=1;i<=m;i++)
scanf("%d%d",&e[i].x,&e[i].y);
sort(e+1,e+1+m,cmp);
for(int i=1;i<=m;i++)
{
if(n>=e[i].x)
{
ans+=e[i].x*e[i].y;
n=n-e[i].x;
}
else
{
ans+=e[i].y*n;
break;
}
}
printf("%d\n",ans);;
return 0;
}