#coding=UTF-8

mojir=input()
hyo=mojir.split(" ")
K=int(hyo[0])
N=int(hyo[1])
F=int(hyo[2])

mojir=input()
hyo=mojir.split(" ")
A=list(range(0,F,1))
for idx in range(0,F,1):
    A[idx]=int(hyo[idx])

ans=K*N-sum(A)
if ans<0:
    print("-1")
else:
    print(ans)