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 = 0
for i in 0..a[2]
    ans = sum - age[i]
end
if sum >=0 
    puts(sum)
else 
    puts "-1"
end