#include<cstdio>
#include<cstring>
#include<algorithm>

#define fi first
#define se second
#define mkp std::make_pair
using ll=long long;
using std::min;
using std::max;
template<class T> void cmax(T&a,T b){a=max(a,b);}
template<class T> void cmin(T&a,T b){a=min(a,b);}

const int NV=4e5;

namespace xm{
    int p[NV+5],dep[NV+5];
    void _(){
        int N;
        scanf("%d",&N);
        for(int i=1;i<=N;++i) scanf("%d",p+i);
        for(int i=1;i<=N;++i) dep[p[i]]=i;
        for(int i=1;i<N;++i) if(std::abs(dep[i]-dep[i+1])>2){
            puts("NO");
            return;
        }
        puts("YES");
    }
}

int main(){
    xm::_();
    return 0;
}