#include <stdio.h>
int find_pattern(int *mem, int n) {
int pos = 0;
int cnt = 0;
for(int i=0; i<n-1; i++){
pos++;
if((*mem + 1) == *(mem + 1)){
cnt++;
if(cnt >= 2) return pos - cnt;
}else{
cnt = 0;
}
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;
}