use std::io;

fn main() {
    let mut input1 = String::new();
    let mut input2 = String::new();
    
    io::stdin().read_line(&mut input1).unwrap();
    let mut input1 = input1.trim().split_whitespace();
    let k: i32 = input1.next().unwrap().parse().unwrap();
    let n: i32 = input1.next().unwrap().parse().unwrap();
    let f: u32 = input1.next().unwrap().parse().unwrap();
    
    io::stdin().read_line(&mut input2).unwrap();
    let mut ages = input2.trim().split_whitespace();
    
    let mut num_beans: i32 = k * n;
    
    for _ in 0..f {
        num_beans -= ages.next().unwrap().parse::<i32>().unwrap();
        
        if num_beans < 0 {
            println!("-1");
            return;
        }
    }
    
    println!("{}", num_beans);
}