#include <stdio.h>
int find_pattern(int *mem, int n) {
// Write your pointer-based logic here
bool found=false;
for(int i=0;i<n-4;i++){
if((*(mem+i+1)==*(mem+i)+1)&&(*(mem+i+2)==*(mem+i+1)+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;
}