#include<iostream>#include<algorithm>usingnamespace std;intconst MAX_LENGTH =5000;int n, m, sum;structSFarmer{int p, a;}farmer[MAX_LENGTH];boolcompare(const SFarmer& first,const SFarmer& second){return first.p < second.p;}intmain(){
cin >> n >> m;for(int i =0; i < m; i++)
cin >> farmer[i].p >> farmer[i].a;sort(farmer, farmer + m, compare);for(int i =0; i < m; i++)if(n >= farmer[i].a){
sum += farmer[i].a * farmer[i].p;
n -= farmer[i].a;}else{
sum += n * farmer[i].p;break;}
cout << sum;return0;}