#include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
using namespace std;
struct pepole{
    int danjia;
    int chanliang;
}; 
int n,m;

bool cmp(pepole x,pepole y){
    if(x.danjia>y.danjia)return false;
    if(x.danjia==y.danjia){
    	if(x.chanliang<y.chanliang)return false;
	}
	return true;
}
int main(){
    pepole p[110000];
    cin>>n>>m;
	
    for(int i=0; i<m; i++){
        cin>>p[i].danjia>>p[i].chanliang;
    }
    
    sort(p,p+m,cmp);
    
    long long ans=0,money=0;
    for(int i=0;i<m;i++){	
	 
    	for(int j=0;j<p[i].chanliang;j++){
    		ans++;
    		money+=p[i].danjia;
    	
    		if(ans==n){
    			cout<<money;
    			return 0;
			}
		}

	}
	cout<<0;
}