چرا با کد زیر بجای اینکه وقتی بردم، تعداد تلاش‌های من برای برد رو بنویسه، می‌نویسه NAN؟

mag = Math.floor(Math.random() * 15)
var mem, tries, score = 100, mem1;
while ((mem != mag) && (score != 0) && (mem1 != "stop")){
    mem = prompt("enter your guess")
    mem1 = mem
    mem = parseInt(mem)
    tries++;
    score = score - 10
    if (mem == mag){
        console.log("good job, you won by " + tries + " tries and you catch " + score + " score!")
    }else if (mem < mag){
        console.log("your guess is less than magic number!")
    }else{
        console.log("your guess is greayer than magic number!")
    }
    if (score == 0){
        console.log("you lose!!!")
    }
}