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#include<bits/stdc++.h>
using namespace std;
struct mi{
    int price,num;
}a[99999999];
bool cmp(mi x,mi y){
    return x.price<y.price;
}
int main(){
    int sum,n,i,tot=0,cost=0;
    cin>>sum>>n;
    for(i=0;i<n;i++){
        cin>>a[i].price>>a[i].num;
    }
    sort(a,a+n,cmp);
    for(i=0;i<n;i++){
        if(sum<a[i].num) a[i].num=sum;
        cost+=a[i].price*a[i].num;
        sum-=a[i].num;
        if(sum==0) break;
    }
    cout<<cost;
}