样例3没过，但AC了（P3819）

//大佬求解qwq P3819
#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
typedef unsigned long long LL;
LL jans=1e18+10,zrs;
struct sjl{
	LL wz,qrs,qzh;
}bnl[N];
int main()
{
	LL l,n;
	cin>>l>>n;
	for(int i=1;i<=n;i++){
		int rs;
		cin>>bnl[i].wz>>rs;
		bnl[i].qrs=bnl[i-1].qrs+rs;
		zrs+=rs;
	}
	LL s=0;
	for(int i=1;i<=n;i++){
		s+=(bnl[i].qrs-bnl[i-1].qrs)*bnl[i].wz;
	}
	for(int i=1;i<=n;i++){
		int rs=bnl[i].qrs-bnl[i-1].qrs;
		int wz=bnl[i].wz-bnl[i-1].wz;
		LL t1=bnl[i-1].qrs*wz;
		LL t2=rs*wz;
		LL t3=(zrs-bnl[i].qrs)*wz;
		s=s+t1-t2-t3;
		jans=min(jans,s);
	}
	cout<<jans;
	return 0;
}