a = gets
a = a.split(" ")
a = a.map!{|x| x.to_i}
age = gets
age = age.split(" ")
age = age.map!{|x| x.to_i}
sum = a[0]*a[1]
ans = sum
for i in 0..a[2]-1
    ans = ans - age[i]
end
if ans >=0 
    puts(ans)
else 
    puts "-1"
end