#include <stdio.h>
int find_pattern(int *mem, int n) {
int* address = mem;
for(int i=0; i<n; i++) {
if(((*mem + 1) == *(mem+1)) && ((*mem + 2) == *(mem+2))) {
return mem - address;
}
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;
}