Firstdata = input();
FirstSplit = Firstdata.split(" ");
result = int(int(FirstSplit[0])*int(FirstSplit[1]));
kazoku = int(FirstSplit[2]);

kazokumame = input();
kazokumamed = kazokumame.split(" ");

for inp in range(0,kazoku):
	result = result - int(kazokumamed[inp]);

if result < 0:
	result = -1;

print(result);