 #include <stdio.h>
#include <string.h>
#include <algorithm>
const int MAXN = 100005;
const int lim = 10;
int n;
int pos[MAXN], f[MAXN][15];
bool flag;
  inline int min(int a, int b)  {
 if( a < 0 ) return b;
return a < b ? a : b;
 }
int main() {
//freopen("in.txt","r", stdin);
  while( scanf("%d", &n), n) {
flag = false;
for(int i = 0; i < n; i++) {
scanf("%d", pos + i);
if( i >= 2 && pos[i] - pos[i - 2] > lim)
flag = true;
 }
if( flag ) {
puts("-1");
continue;
}
memset(f, -1, sizeof( f));
f[1][ pos[1] - pos[0] ] = 0;
for(int i = 1; i < n; i ++) {
for(int j = 1; j <= lim; j ++) {
for(int k = i + 1; k <= i + lim && k < n; k ++) {
if( f[i][j] >= 0 && pos[k] - (pos[i] - j) <= lim ) {
f[k][ pos[k] - pos[i] ] = min(f[k][ pos[k] - pos[i] ], f[i][j] + 1);
}
}
}
}
int ans = 999999999;
for(int i = 1; i <= lim; i ++) {
ans = min(f[n-1][i], ans);
}
printf("%d\n", ans);
}
//while(1);
return 0;
}
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