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