#include<iostream>#include<stdio.h>#include<string.h> using namespace std;enum { SIZ = 10008,};int num;int pos[SIZ];bool fun(){ int i, t; int lim = (num+1)/2; for(t=1; t<lim; t++){ for(i=t*2; i<num; i++){ if(pos[i-t] < pos[i]){ if(pos[i-t-t]< pos[i-t]) return false; }else if(pos[i-t] > pos[i]){ if(pos[i-t-t]> pos[i-t]) return false; } } } return true;}int readIn(){ char ch; scanf("%d ", &num); if(num ==0) return 0; scanf("%c", &ch); int i, t; for(i=0; i<num; i++){ scanf("%d", &t); pos[t] = i; } return num;}int main(){ const char *ans[2] = { "no", "yes" }; while( readIn() > 0){ printf("%sn", ans[fun()]); } return 0;}
