#include<bits/stdc++.h>
using namespace std;const int N=1e5+7;
int n,m,ans=1e9,minn=1e9,fl=1,a[N],b[N],c[N];
int sum(int x){return a[c[x]]-x;}
int main(){
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;++i){scanf("%d",&a[i]);b[i]=a[i];if(a[i]>a[i-1])fl=0;}
	if(fl){puts("0");return 0;}sort(b+1,b+n+1);
	for(int i=1;i<=n;++i)c[lower_bound(b+1,b+n+1,a[i])-b]=i;
	for(int r=1,l=1;r<n;++r){
		if(c[r]<c[r-1])l=r;int x=max(2*(l-1)-1,2*(n-r));
		if((n-sum(l)-1>=x&&sum(r)-sum(l)>=n-1-r)||(sum(r)>=x&&sum(r)-sum(l)>=l-1))
		ans=min(ans,x);
	}
	printf("%d\n",ans==1e9?-1:ans+1);
}