思路：
三分；
代码：
#include <bits/stdc++.h>
using namespace std;
#define maxn 100005
#define ll long long
struct 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=;
while(Cget>''||Cget<'')Cget=getchar();
while(Cget>=''&&Cget<='')
{
now=now*+Cget-'';
Cget=getchar();
}
}
ll lower(ll key)
{
ll l=,r=n,res=,mid;
while(l<=r)
{
mid=l+r>>;
if(key<=ai[mid].pi) res=mid,r=mid-;
else l=mid+;
}
return res;
}
ll calculate(ll key)
{
ll now=lower(key);
ll res1=key*ai[now-].sumval-ai[now-].sumpi;
ll res2=ai[n].sumpi-ai[now-].sumpi-key*(ai[n].sumval-ai[now-].sumval);
return res2+res1;
}
int main()
{
in(len),in(n);
for(ll i=;i<=n;i++) in(ai[i].pi),in(ai[i].val);
sort(ai+,ai+n+);
for(ll i=;i<=n+;i++)
{
ai[i].sumval=ai[i-].sumval+ai[i].val;
ai[i].sumpi=ai[i-].sumpi+ai[i].val*ai[i].pi;
}
ll l=,r=len,mid,now,nowl,nowr;
while(l<r)
{
mid=l+r>>;
now=calculate(mid);
nowl=calculate(mid-);
nowr=calculate(mid+);
if(nowl<now) r=mid-;
else if(nowr<now) l=mid+;
else
{
printf("%lld\n",calculate(mid));
return ;
}
}
printf("%lld\n",calculate(l));
return ;
}