import java.util.*;
public class Main 
{
	public static void main(String[] args)
	{
		Scanner cin = new Scanner(System.in);
		int a,b,m,total,c=0,cm,ta;
		double result=0;
		while(cin.hasNext())
		{
			m=cin.nextInt();
			result=0;
			c=ta=0;
			int d[]=new int[m];
			for(int i=0;i<m;i++)
				d[i]=0;
			total=cin.nextInt();
			for(int i=0;i<total;i++)
			{
				a=cin.nextInt();
				cm=25;
				for(int j=0;j<m;j++)
				{
					d[j]=d[j]-(a-ta);
					if(d[j]<cm)
					{
						cm=d[j];
						c=j;
					}
				}
				b=cin.nextInt();
				if(d[c]<1)
				{
					d[c]=b;
				}
				else
				{
					result=result+d[c];
					d[c]=d[c]+b;
				}
				ta=a;
			}
			System.out.printf("%.2f\n",result/total);
		}
		cin.close();
	}
}
/**************************************************************
	Problem: 2147
	User: admin
	Language: Java
	Result: Accepted
	Time:762 ms
	Memory:41452 kb
****************************************************************/