usaco milk

#include<stdio.h>int sum,n,v[6001],num[6001];void qs(int l,int r){int i,j,mid,t;i=l;j=r;mid=v[(l+r)/2];while (i<j) {while (v[i]<mid) i++;while (v[j]>mid) j--;if (i<=j) {t=v[i];v[i]=v[j];v[j]=t;t=num[i];num[i]=num[j];num[j]=t;i++;j--;}}if (l<j) qs(l,j);if (i<r) qs(i,r);}int main(){freopen("milk.in","r",stdin);freopen("milk.out","w",stdout);int i,j,ans=0;scanf("%d%d",&sum,&n);for (i=1;i<=n;i++) scanf("%d%d",&v[i],&num[i]);qs(1,n);for (i=1;i<=n;i++) {if (sum>num[i]) {ans+=v[i]*num[i];sum-=num[i];}else {ans+=sum*v[i];break;}}printf("%d\n",ans);return 0;}