let array = gets().split(' ').map(Number);
let isWaveForm = false;

for(let i = 1; i <= array.length; i+=2){
    let currentNum = array[i];
    let previousNum = array[i-1];
    let nextNum = array[i+1];

    if(currentNum >= previousNum || currentNum >= nextNum){
        isWaveForm = true;
    }else{
        isWaveForm = false;
    }
}
if(!isWaveForm){
    console.log('yes');
}else{
    console.log('no');
}