/*ID:kevin_s1PROG:milkLANG:C++*/#include 
     
      #include 
      
       #include 
       
        #include 
        
         #include 
         
          #define MAXN 5001using namespace std;int N,M;struct farmer{	int Pi;	int Ai;}farmers[MAXN];int cmp(farmer a,farmer b){	return a.Pi < b.Pi;}int main(){	freopen("milk.in","r",stdin);	freopen("milk.out","w",stdout);	cin>>N>>M;	for(int i = 1; i <= M; i++){		cin>>farmers[i].Pi>>farmers[i].Ai;	}	sort(farmers + 1, farmers + M + 1, cmp);	int count = 0;	int index = 1;	int money = 0;	while(count < N){		if(count + farmers[index].Ai <= N){			count = count + farmers[index].Ai;			money += farmers[index].Pi * farmers[index].Ai;			index++;		}		if(count + farmers[index].Ai > N){			money += farmers[index].Pi * (N - count);			count = N;			index ++;		}	}	cout<
          
           <
          
         
        
       
      
     

//greedy algorithm