#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// Yukicoder №123 カードシャッフル
// https://yukicoder.me/problems/no/123
// ***********************
// for debug
#define DEBUG

#define NOP do{}while(0)
#ifdef DEBUG
#define TRACE(...) do{printf(__VA_ARGS__);fflush(stdout);}while(0)
#define TRACECR do{printf("\n");fflush(stdout);}while(0)
#else
#define TRACE(...) NOP
#define TRACECR NOP
#endif

#define PRINCR printf("\n")
#define NOCR(strig) do{char *p;p=strchr(strig,'\n');if(p)*p='\0';}while(0)
// The out-of-date function
#define	asctime(...)	asctime_s(...)
#define	ctime(...)		ctime_s(...)
#define	strlen(a)	mystr_len(a)

// for stdio
#define	GETLINE	INPUT
#define INPUT(str) do{char *p;fgets(str,sizeof(str),stdin);p=strchr(str,'\n');if(p)*p='\0';}while(0)
#define REP(a,b) for(int a=0;a<(int)(b);++a)
#define lolong long long
#define INPBUF (250+2)
static char *GETWORD(char* str) {char c;char *cp;cp=&str[0];c=fgetc(stdin);while( c != EOF ){if((c==' ')||( c=='\n')) break;*cp++=c;c=fgetc(stdin);}*cp='\0';return &str[0];}
#define	GETINTS(a,b) {char s[257];int *ap=a;REP(i,b){GETWORD(s);sscanf(s,"%d", ap);ap++;}}
// ***********************
// ***********************
// 外部変数
int	C[100000];
// ***********************
int main() {
	int top;
	int	N,M;
	int	C[100000];
	char	str[51];

	GETLINE(str);
	sscanf( str,"%d %d", &N, &M );

	GETINTS( C, M );

	top = C[M-1];
	for (int i = M - 2; i >= 0; --i)
	{
		if( top == 1 ) {
			top = C[i];
		} else {
			if(top <= C[i] ) top--;
		}
	}
	printf("%d\n", top);
	
}