#include<bits/stdc++.h> using namespace std; struct sb { int danjia; int huocunliang; } a[100]; bool cmp(sb a,sb b) { if(a.danjia<b.danjia) { return true; } else { return false; } } int main() { int m,n; cin>>m>>n; for(int i=0; i<n; i++) { cin>>a[i].danjia>>a[i].huocunliang; } sort(a,a+n,cmp); int sum=0,price=0; for(int i=0; i<n; i++) { sum+=a[i].huocunliang; price +=(a[i].huocunliang*a[i].danjia); if(sum>m) { price-=(sum-m)*a[i].danjia; break; } } cout<<price; }