usaco milk

#include<stdio.h>#include<stdlib.h>int p[5010],a[5010],sum=0;int main(){int n,m;freopen("milk.in","r",stdin);freopen("milk.out","w",stdout);scanf("%d%d",&n,&m);int i,j,k;for(i=1;i<=m;i++)  scanf("%d%d",&p[i],&a[i]);for(i=1;i<m;i++)  for(j=i+1;j<=m;j++)    if(p[i]>p[j]){    k=p[i];    p[i]=p[j];    p[j]=k;    k=a[i];    a[i]=a[j];    a[j]=k;    }i=0;while(n>0){i++;if(a[i]<=n){n-=a[i];sum+=a[i]*p[i];}else{sum+=n*p[i];n=0;}}printf("%d\n",sum);return 0;}