E

#include <iostream>

using namespace std;

int main()
{
    int n;
    cin >> n;
    int l;
    cin >> l;
    bool flag = false;
    while(--n){
        int c;
        cin >> c;
        if(c*l > 0 )
            flag = true;
        l = c;
    }
    cout << (flag ? "YES" : "NO");
    return 0;
}