rt，第三个样例没过，但AC,建议加强数据

code：

#include <bits/stdc++.h>
#define ll long long
using namespace std;

ll a[100005];
ll b[100005];
ll cnt=0;
struct people{
    ll x,r;
}p[100005];
bool cmp(people a,people b)
{
    return a.x<b.x;
}
int main()
{
    ll l,n;
    scanf("%lld%lld",&l,&n);
    for(int i=1;i<=n;i++)
        scanf("%lld%lld",&p[i].x,&p[i].r);
    sort(p+1,p+n+1,cmp);
    for(int i=1;i<=n;i++)
    {
        a[i]=a[i-1]+cnt*(p[i].x-p[i-1].x);
        cnt+=p[i].r;
    }
    cnt=0;
    ll ans=LONG_MAX;
    for(int i=n;i>=1;i--)
    {
        b[i]=b[i+1]+cnt*(p[i+1].x-p[i].x);
        ans=min(ans,b[i]+a[i]);
        cnt+=p[i].r;
    }
    printf("%lld\n",ans);
    return 0;
}