#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int *end = mem + n -1;
int *start = mem;
while(mem < end){
if(*(mem)+1 == *(mem+1) && *(mem+1)+1 == *(mem+2))
return (mem - start);
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;
}