#include <stdio.h>
int find_pattern(int *mem, int n) {
int *p1 = mem; // points to current
int *p2 = mem + 1; // points to next
int *p3 = mem + 2; // points to next+1
for (int i = 0; i <= n - 3; i++) {
if (*p1 + 1 == *p2 && *p2 + 1 == *p3) {
return i; // found increasing pattern
}
p1++;
p2++;
p3++;
}
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;
}