#include<bits/stdc++.h>
using namespace std;
struct Milk{
    int val,w;
}milk[5005];
bool cmp(milk &x,milk &y){return x.w<=y.w}
int n,m,ans=0;
int main(){
    scanf("%d%d",&n,&m);
    for(int i=1;i<=m;i++) scanf("%d%d",&milk[i].w,&milk[i].val);
    sort(milk+1,milk+m+1,cmp);
    for(int i=1;i<=m;i++){
        if(n==0) break;
        if(milk[i].val>=n){ans+=milk[i].w*n;n=0;}
        else{ans+=milk[i].w*milk[i].val;n-=milk[i].val;}
    }
    printf("%d",ans);
    return 0;
}

RE点6