#include <bits/stdc++.h>using namespace std;#define maxn 100005#define ll long longstruct DataType{    ll pi,val,sumval,sumpi;    bool operator<(const DataType pos)const    {        return pi<pos.pi;    }};struct DataType ai[maxn];ll len,n;inline void in(ll &now){    char Cget=getchar();now=0;    while(Cget>‘9‘||Cget<‘0‘)Cget=getchar();    while(Cget>=‘0‘&&Cget<=‘9‘)    {        now=now*10+Cget-‘0‘;        Cget=getchar();    }}ll lower(ll key){    ll l=1,r=n,res=1,mid;    while(l<=r)    {        mid=l+r>>1;        if(key<=ai[mid].pi) res=mid,r=mid-1;        else l=mid+1;    }    return res;}ll calculate(ll key){    ll now=lower(key);    ll res1=key*ai[now-1].sumval-ai[now-1].sumpi;    ll res2=ai[n].sumpi-ai[now-1].sumpi-key*(ai[n].sumval-ai[now-1].sumval);    return res2+res1;}int main(){    in(len),in(n);    for(ll i=1;i<=n;i++) in(ai[i].pi),in(ai[i].val);    sort(ai+1,ai+n+1);    for(ll i=1;i<=n+1;i++)    {        ai[i].sumval=ai[i-1].sumval+ai[i].val;        ai[i].sumpi=ai[i-1].sumpi+ai[i].val*ai[i].pi;    }    ll l=0,r=len,mid,now,nowl,nowr;    while(l<r)    {        mid=l+r>>1;        now=calculate(mid);        nowl=calculate(mid-1);        nowr=calculate(mid+1);        if(nowl<now) r=mid-1;        else if(nowr<now) l=mid+1;        else        {            printf("%lld\n",calculate(mid));            return 0;        }    }    printf("%lld\n",calculate(l));    return 0;}