#include <stdio.h>

using namespace std;
int n,i,k,x,y,grup,r,gr;
FILE *f,*g;
int main()
{f=fopen ("grupuri.in","r");
 g=fopen ("grupuri.out","w");
 fscanf (f,"%d%d",&k,&n);
 for (i=1;i<=n;i++)
 {
     fscanf (f,"%d",&x);
     grup=(n-i+1)*(x-y)+r;
     r=grup%k;
     grup/=k;
     gr+=grup;
     y=x;
 }
 fprintf (g,"%d",gr);
    return 0;
}