#include <stdio.h>
int find_pattern(int *mem, int n) {
int start_index=0,found = 0;
while(!found && start_index<=n-3){
if((((*mem)+1) == (*(mem+1))) && (((*(mem+1))+1) == (*(mem+2)))){
found = 1;
}
if(found){
return start_index;
}
start_index+=1;
mem++;
}
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;
}