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