#include <iostream>
using namespace std;
 
int main() {
	int odleglosc,l,pom=0,suma=0;
	cin>>odleglosc>>l;
	pom=odleglosc/800;
 
 
	int tab2[l],tab[l];
	for(int i=0; i<l;i++)
	{
		cin>>tab[i]>>tab2[i];
	}
	int n=0,a=800,d=0,min,pom2=0,z=0;
	while(z<l)
	{
 
			while(tab[n]<=a)
			{
				if(n>=l) break;
				else
				n++;
 
			}
 
			a=a+800;
 
		min=tab2[d];
					for(int i=d;i<n;i++)
					{
						z++;
						if(min>tab2[i])
						min=tab2[i];
					}
 
					for(int j=d;j<n;j++)
					{	
						if(min==tab2[j])
						pom2=j;
					}
 
				suma=suma+min;
 
			d=n;
		n=pom2;
	}
				cout<<suma;
 
 
}
 
 
 
 

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

MjAwMCAyMAoKMTAwIDEyCjIwMCAyNAozMDAgNTIKNDAwIDEKNTAwIDI0NQo2MDAgOTAwCjcwMCAxMjQKODAwIDEyNQo5MDAgMTIzCjEwMDAgMjUKMTEwMCA3NQoxMjAwIDk5OQoxMzAwIDc2MwoxNDAwIDUxMgoxNTAwIDU0CjE2MDAgMTIzCjE3MDAgNTQKMTgwMCAyMwoxOTAwIDEyMwoyMDAwIDQy

2000 20

100 12
200 24
300 52
400 1
500 245
600 900
700 124
800 125
900 123
1000 25
1100 75
1200 999
1300 763
1400 512
1500 54
1600 123
1700 54
1800 23
1900 123
2000 42