#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
int diff1, diff2;
for (int i=0;i<n-2;i++){
diff1 = *(mem+i+1) - *(mem+i);
diff2 = *(mem+i+2) - *(mem+i+1);
if (diff1==1){
if (diff2==1){
return i;
}
}
}
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;
}