#coding: utf-8
##yuki_143

m=[]
a=[]
m=map(int,raw_input().split())
a=map(int,raw_input().split())

k=m[0]
n=m[1]
f=m[2]

kn=k*n

sum=0
for i in xrange(len(a)):
 sum+=a[i]

res=kn-sum

if res<0:
 print -1
else:
 print res