#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int temp = 0,count =0;
for(int i=0;i<n-2;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;
}