#include <stdio.h>
int find_pattern(int *mem, int n) {
int pos = 0;
for(int i=0; i<n-3; i++){
if((*mem)+1 == *(mem+1) && (*(mem+1)+1) == *(mem+2)) {
int count = pos;
return count;
}
pos++;
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;
}