#include<stdio.h>
int n,k;
int a[100000+10];

void open(){
	freopen("teststr.in","r",stdin);
	freopen("teststr.out","w",stdout);
}

void read(){
	scanf("%d %d",&n,&k);
	for(int i=1;i<=k;i++){
		scanf("%d",&a[i]);
	}
}

int solve(){
	int ans_true=n;
	int ans_false=n;
	for(int i=1;i<=k;i++)if(a[i]<ans_true)ans_true=a[i];
	for(int i=1;i<=k;i++)if((n-a[i])<ans_false)ans_false=a[i];
	return ans_true>ans_false? ans_true:ans_false;
}

int main(){
	open();
	read();
	int ans=solve();
	printf("%d",ans);
	return 0;
}