#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
mem++;
for(int i=0;i<n;i++) {
if( (*(mem+1))== ((*mem)+1) && (*(mem-1))==((*mem)-1) )
return i;
mem++;
}
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;
}