#include <iostream.h>
#include <stdlib.h>

#define MAX 130001


long t[MAX],s[MAX];
long n,i,tmp,j,tgcd;

long gcd(long a, long b){
  long tmp;
  while (b>0){
     tmp = a%b;
     a=b; b=tmp;
  }
  return a;
}


int main(){

cin >> n;
for (i=0;i<MAX;++i) s[i]=0;

for (i=1; i<=n;++i){
  cin >>tmp; s[tmp]=i;
}
j=0;
for (i=1;i<MAX; ++i)
  if (s[i]) t[++j]=s[i];

tgcd = t[1]-1;

for (j=2;j<=n;++j){
  if (t[j]>j)
   tgcd=gcd(tgcd,t[j]-j);
  if (t[j]<j) tgcd = gcd(tgcd,j-t[j]);

 }
tgcd = (tgcd) ? tgcd-1 : 0;
cout << tgcd<<endl;

return 0;
}