#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int step = 0;
while( step <= n-1){
int * first = mem+step;
int * second = mem+step+1;
if(*second - *first == 1){
int * third = mem+step+2;
if(*third - *second == 1){
return step;
}
step++;
continue;
}
step++;
continue;
}
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;
}