[рони]

eridan,

function next(b, c) {
    for (var d = c / b * 100 | 1, a = 1; a++;)
        if (((c + a) / (b + a) * 100 | 0) >= d) return a
};
var all_b = 993, pobed = 519,n =  next(all_b, pobed);
alert(n)