import std.stdio, std.conv, std.string, std.range, std.array, std.algorithm;

int main(){
	auto I = readln.strip.split.to!(int[]);
	auto A = I[0]*I[1] - readln.strip.split.to!(int[]).sum;
	(A<0 ? -1 : A).writeln;
	return 0;
}