#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int *p1 = mem;
int *p2 = mem+1;
int *p3 = mem+2;
for (int j=0; j<n-3; j++) {
if (*p1+1==*p2 && *p2+1==*p3)
return j;
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;
}