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