#include<stdio.h>

int a[6000], lung, i, j, min, max, n, cnt;

int main(){
    freopen("secv.in","r",stdin);
    freopen("secv.out","w",stdout);
    scanf("%d",&n);
    max = 0; lung = min = 2000000000;
    for (i = 1; i <= n; i++){
        scanf("%d",&a[i]);
        if (a[i]> max) max = a[i];
        if (a[i]< min) min = a[i];
    }
    for (i = 1; i <= n; i++)
        if (a[i]==min){
            cnt = min+1;
            for (j=i+1;j<=n && cnt <= max ;j++)
                if (a[j] == cnt) cnt++;
            if (j-i < lung && cnt == max+1)
                lung = j-i;
        }
    printf("%d\n",(lung < 200000000 ? lung : -1));
    return 0;
}