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