#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int cons = 0;
for(int i = 0; i<n-1; i++){
if(*(mem)+1 == *(mem+1)){
cons++;
if(cons == 2) return i-1;
}else{
cons = 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;
}