#include<cstdio>
int a[5001],p[5001],i,j,k,m,n,b[5001],c,l;
int main() {
	freopen("secv.in","r",stdin),freopen("secv.out","w",stdout),scanf("%d",&n),a[0]=-1;
	for(i=1;i<=n;i++) {
		scanf("%d",a+i);
       	if(a[b[m]]<a[i])
        	p[i]=b[m],b[++m]=i;
       	for(j=0,k=m;j<k;)
		if(a[b[(j+k)/2]]<a[i])
            j=(j+k)/2+1;
        else
            k=(j+k)/2;
       	if(a[i]<a[b[j]]) {
		   	if(j)
                p[i]=b[j-1];
            b[j]=i;
		}
	}
	for(i=m,j=b[m];i--;j=p[j])
    	b[i]=j;
    for(l=m-1;l;) {
		for(c=l,i=b[l]-1;i>b[0];i--) {
			while(b[c]>i&&c)
				c--;
			if(b[c]!=i) {
				for(j=0;j<=l&&a[b[j]]!=a[i];j++);
				if(j>l)
					l=c;
			}
		}
	}
    printf("%d",!l?-1:(b[l]-b[0]+1));
}