//No.143 豆

import java.util.*;
import java.io.*;
import static java.util.Arrays.*;
import static java.lang.Math.*;

public class No143 {

    static final InputStream in = System.in;
    static final PrintWriter out = new PrintWriter(System.out,false);

    static void solve(){
        int k = nextInt();
        int n = nextInt();
        int f = nextInt();
        int sum = 0;
        for (int i=0; i<f; i++) {
            sum += nextInt();
        }

        out.println(k*n < sum ? "-1" : (k*n - sum));
    }

    public static void main(String[] args) throws Exception {
        long start = System.currentTimeMillis();

        solve();
        out.flush();

        long end = System.currentTimeMillis();
        //trace(end-start + "ms");
        in.close();
    }

    static void trace(Object... o) { System.out.println(deepToString(o));}

    static final byte[] buf = new byte[1024];
    static int ptr = 0;
    static int buflen = 0;

    static boolean hasNextByte() {
        if (ptr < buflen)
            return true;
        ptr = 0;
        try{
            buflen = in.read(buf);
        }catch (IOException e) {e.printStackTrace();}
        if (buflen <= 0)
            return false;
        return true;
    }

    static int readByte() { if (hasNextByte()) return buf[ptr++]; else return -1;}

    static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}

    static void skip() { while(hasNextByte() && !isPrintableChar(buf[ptr])) ptr++;}

    static boolean hasNext() {skip(); return hasNextByte();}

    static String next() {
        if (!hasNext()) throw new NoSuchElementException();
        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (isPrintableChar(b)) {
            sb.appendCodePoint(b);
            b = readByte();
        }
        return sb.toString();
    }

    static long nextLong() {
        if (!hasNext()) throw new NoSuchElementException();
        boolean minus = false;
        long num = readByte();

        if(num == '-'){
            num = 0;
            minus = true;
        }else if (num < '0' || '9' < num){
            throw new NumberFormatException();
        }else{
            num -= '0';
        }
        
        while(true){
            int b = readByte();
            if('0' <= b && b <= '9')
                num = num * 10 + (b - '0');
            else if(b == -1 || !isPrintableChar(b))
                return minus ? -num : num;
            else
                throw new NoSuchElementException();
        }
    }

    static int nextInt() {
        long num = nextLong();
        if (num < Integer.MIN_VALUE || Integer.MAX_VALUE < num)
            throw new NumberFormatException();
        return (int)num;
    }

    static double nextDouble() {
        return Double.parseDouble(next());
    }

    static char nextChar() {
        if (!hasNext()) throw new NoSuchElementException();
        return (char)readByte();
    }

    static String nextLine() {
        while (hasNextByte() && (buf[ptr] == '\n' || buf[ptr] == '\r')) ptr++;
        if (!hasNextByte()) throw new NoSuchElementException();

        StringBuilder sb = new StringBuilder();
        int b = readByte();
        while (b != '\n' && b != '\r') {
            sb.appendCodePoint(b);
            b = readByte();
        }

        return sb.toString();
    }

    static int[] nextArrayInt(int n) {
        int[] a = new int[n];
        for (int i=0; i<n; i++) a[i] = nextInt();
        return a;
    }
}