#include<bits/stdc++.h> using namespace std; struct milk{ int mn,num; }a[2000010]; bool cmp(milk n1,milk n2){ if(n1.mn<n2.mn){ return 1; }else{ return 0; } } int n,m,sum,c; int main(){ cin>>m>>n; for(int i=1;i<=n;i++){ cin>>a[i].mn>>a[i].num; } sort(a+1,a+1+n,cmp); for(int i=1;i<=n;i++){ if(c+a[i].num<m){ c=c+a[i].num; sum=sum+a[i].mn*a[i].num; }else{ sum=sum+(m-c)*a[i].mn; break; } } cout<<sum; return 0; }