record

#include<bits/stdc++.h>
using namespace std;
struct milk{
	int p,a;
} s[2000005];
int n,m,ans,ln;
bool cmp(milk x,milk y){
	return x.p<y.p;
}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=0;i<m;++i){
		scanf("%d%d",&s[i].p,&s[i].a);
	}
	sort(s,s+m,cmp);
	for(int i=0;i<m;++i){
		if(ln>s[i].a) ans+=s[i].p*s[i].a,ln-=s[i].a;
		else{
			ans+=ln*s[i].p;break;
		}
	}
	printf("%d",ans);
	return 0;
}

24pts，求助大佬！