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