import std.stdio;
import std.algorithm;
import std.conv;
import std.range;

void main()
{
    auto input = readln.splitter.map!(to!int).array;
    immutable numBeans = input[0] * input[1];
    immutable totalAge = readln.splitter.map!(to!int).sum;
    writeln(numBeans >= totalAge ? numBeans - totalAge : -1);
}