USACO 1.3.1 Mixing Milk

/*
ID:wconvey
PROG:milk
LANG:C++
*/
#include<iostream>
#include<cstdio>
using namespace std;

struct node{
int p,a;
}array[5100];

int cmp(const void *x,const void *y)
{
if(((struct node *)x)->p!=((struct node *)y)->p)
return  ((struct node *)x)->p-((struct node *)y)->p;
else
return ((struct node *)x)->a-((struct node *)y)->a;
}

int main(){
freopen("milk.in","r",stdin);
freopen("milk.out","w",stdout);
int i,n,m,ans=0,cnt;
scanf("%d %d",&n,&m);
for(i=0;i<m;i++)
scanf("%d %d",&array[i].p,&array[i].a);
qsort(array,m,sizeof(array[0]),cmp);

cnt=n;
for(i=0;cnt!=0;i++)
{
if(cnt>array[i].a)
{
cnt-=array[i].a;
ans+=array[i].p*array[i].a;
}
else{
ans+=array[i].p*cnt;
cnt=0;
}
}
printf("%d\n",ans);
return 0;
}