/*
ID: mytbk921
LANG: C
TASK: barn1
*/

#include <stdio.h>
#define MAXS 200

int main()
{
	int MBoards,nStalls,nCows;
	int l,r,ngaps,occupied;
	int stalls[MAXS+10]={},gaps[MAXS];
	int i,j;

	FILE *fin=fopen("barn1.in","r");
	FILE *fout=fopen("barn1.out","w");
	fscanf(fin,"%d%d%d",&MBoards,&nStalls,&nCows);
	l=nStalls; r=0; //initial the left&right bound
	for (i=0;i<nCows;++i){
		fscanf(fin,"%d",&j);
		stalls[j]=1;
		if (j<l) //update the bound
			l=j;
		if (j>r)
			r=j;
	}
	fclose(fin);

	ngaps=0;
	for (i=l;i<=r;i++){
		if (!stalls[i]){ //have a gap
			for (j=i;!stalls[j];j++)
				;
			gaps[ngaps++]=j-i;
			i=j; //the i will continue
		}
	}
	//selection sort
	for (i=0;i<ngaps&&i<MBoards-1;i++){ //select the largest gaps
		int max=i; //form gaps[i] to gaps[ngaps-1]
		for (j=i+1;j<ngaps;j++){
			if (gaps[j]>gaps[max])
				max=j;
		}
		int tmp=gaps[max];
		gaps[max]=gaps[i];
		gaps[i]=tmp; //swap finished
	}

	//calculate
	occupied=r-l+1;
	for (i=0;i<MBoards-1&&i<ngaps;i++)
		occupied-=gaps[i];
	
	fprintf(fout,"%d\n",occupied);
	return 0;
}