#include <stdio.h>
int find_pattern(int *mem, int n) {
if(n<3) return -1;
int count=0;
int *start = mem;
int *end = mem + n-2;
while(mem<end){
if((*(mem + 1) == *mem + 1))count++;
else count=0;
if (count==2) return (mem-1)-start;
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;
}