#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
if(n<3)
return -1;
else{
int idx=0,prev=*mem,count=1;
for(int i=1;i<n;i++){
if (*(mem+i)==++prev){
prev=*(mem+i);
if(++count==3)
return idx;
continue;}
idx=i;
prev=*(mem+i);
}
return -1;
}
}
int main() {
int n, arr[100];
scanf("%d", &n);
for (int i = 0; i < n; i++) {
scanf("%d", &arr[i]);
}
int res = find_pattern(arr, n);
printf("%d", res);
return 0;
}