using System;

public class Program

{ public static int days()

{int s,m,n,i,d=0,t=0,td,r;

 Console.Write("total distance:");

 s=int.Parse(Console.ReadLine());

 Console.Write("max distance covered in 1 day:");

 m=int.Parse(Console.ReadLine());

 Console.Write("exceptions:");

 n=int.Parse(Console.ReadLine());

 if((s>0)&&(m>0)&&(n>=0))

 {int[] x=new int[n];

 int[] y=new int[n];

 for(i=0;i<n;i++)

    { Console.Write("day:");x[i]=int.Parse(Console.ReadLine());

     Console.Write("distance covered on "+x[i]+"th day");y[i]=int.Parse(Console.ReadLine());

    t=t+y[i];

    }

 d=n;

 r=s-t;

td=d+(r/m);

 if((r%m)>0)

 {td=td+1;   

return td;}

 else

 {return td;}}

 else

 {return -1;}

}

    public static void Main()

    {int p;

     p=days();

     if(p==-1)

     {Console.WriteLine("invalid input\n");

     Main();}

     else{

        Console.WriteLine("\ndays taken:{0}",p);}

    }

}