#include <cstdio>

using namespace std;

int v[5000];

int main(){
    freopen("secventa.in","r",stdin);
    freopen("secventa.out","w",stdout);
    int n,i,min,max,j,lun,poz,poz2,last;
    scanf("%d",&n);
    min = 2000000000;
    max = 0;
    for (i=0;i<n;i++){
        scanf("%d",&v[i]);
        if (v[i] < min)
          min = v[i],poz = i;
        if (v[i] > max)
          max = v[i];
    }
    poz2 = poz;
    while (min < max && last != min){
      last = min;
      min = max;
      for (i=poz2+1;i<n;i++)
        if (v[i] < min && v[i] > last)
          min = v[i],poz2 = i;
    }
    if (min == max)
      printf("%d",poz2 - poz + 2);
    else
      printf("-1");
    return 0;
}