这是代码

#include<cstdio>
#include<algorithm>

struct milk{
	int p,a;
} a[5005];

bool cmp(milk x,milk y){
	return x.p<y.p;
}

int main(){
	int n,m,ans=0;
	scanf("%d %d",&n,&m);
	for(int i=0;i<m;i++)
		scanf("%d %d",&a[i].p,&a[i].a);
	std::sort(a,a+n,cmp);
	for(int i=0;n>0;){
		if(a[i].a!=0){
			a[i].a--;
			n--;
			ans+=a[i].p;
		} else
			i++;
	}
	printf("%d",ans);
	return 0;
	}