We have had record hot temperatures this summer. To avoid heat stroke, you decided to buy a quantity of drinking water at the nearby supermarket. Two types of bottled water, 1 and 0.5 liter, are on sale at respective prices there. You have a definite quantity in your mind, but are willing to buy a quantity larger than that if: no combination of these bottles meets the quantity, or, the total price becomes lower.

Given the prices for each bottle of water and the total quantity needed, make a program to seek the lowest price to buy greater than or equal to the quantity required.



Input

The input is given in the following format.


$A$ $B$ $X$


The first line provides the prices for a 1-liter bottle $A$ ($1\leq A \leq 1000$), 500-milliliter bottle $B$ ($1 \leq B \leq 1000$), and the total water quantity needed $X$ ($1 \leq X \leq 20000$). All these values are given as integers, and the quantity of water in milliliters.

Output

Output the total price.

Examples

Input

180 100 2400


Output

460


Input

200 90 2018


Output

450
import java.util.*;
import java.util.stream.*;
import java.io.*;
import java.math.*;

public class Main {

	static void solve (FastScanner in, PrintWriter out, Methods ms) {
		
		int A = in.nextInt(), B = in.nextInt(), goal = in.nextInt();
		
		if (2*B < A) {
			out.println((goal+500-1)/500*B);
		}
		else {
			int ans = goal / 1000 * A;
			goal %= 1000;
			
			if (goal == 0) {
				out.println(ans);
			}
			else {
				out.println(ans + (goal<=500? Math.min(A, B) : Math.min(A, B*2)));
			}
			
		}
		
	}
	


	public static void main(String[] args) {
		FastScanner in = new FastScanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		Methods ms = new Methods();
		solve(in, out, ms);
		in.close();
		out.close();
	}

	static class Methods {

		public void print (Object... ar) {System.out.println(Arrays.deepToString(ar));}

		public int max (int... ar) {Arrays.sort(ar); return ar[ar.length-1];}

		public int min (int... ar) {Arrays.sort(ar); return ar[0];}

		public long gcd (long a, long b) {return b>0?gcd(b,a%b):a;}

		public long lcm (long a, long b) {return a/gcd(a,b)*b;}

		public long factorial (int i) {
			if (i==1) return 1;
			else return i*factorial(i-1);
		}

		public int manhat (int x1, int y1, int x2, int y2) {
			return Math.abs(x1-x2)+Math.abs(y1-y2);
		}

		public double euclid (double x1, double y1, double x2, double y2) {
			return Math.sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
		}

		public boolean isPrime (int n) {
			if (n==2) return true;
			if (n<2 || n%2==0) return false;
			for (int i=3; i<=Math.sqrt(n); i+=2) if(n%i==0) return false;
			return true;
		}

	}

	static class FastScanner {

		private InputStream in;
		private byte[] buffer = new byte[1024];
		private  int length = 0, p = 0;

		public FastScanner (InputStream stream) {
			in = stream;
		}

		public boolean hasNextByte () {
			if (p < length) return true;
			else {
				p = 0;
				try {length = in.read(buffer);}
				catch (Exception e) {e.printStackTrace();}
				if (length <= 0) return false;
			}
			return true;
		}

		public int readByte () {
			if (hasNextByte() == true) return buffer[p++];
			return -1;
		}

		public boolean isPrintable (int n) {return 33<=n&&n<=126;}

		public void skip () {
			while (hasNextByte() && !isPrintable(buffer[p])) p++;
		}

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

		public String next () {
			if (!hasNext()) throw new NoSuchElementException();
			StringBuilder sb = new StringBuilder();
			int t = readByte();
			while (isPrintable(t)) {
				sb.appendCodePoint(t);
				t = readByte();
			}
			return sb.toString();
		}

		public String[] nextArray (int n) {
			String[] ar = new String[n];
			for (int i=0; i<n; i++) ar[i] = next();
			return ar;
		}

		public int nextInt () {return Math.toIntExact(nextLong());}

		public int[] nextIntArray (int n) {
			int[] ar = new int[n];
			for (int i=0; i<n; i++) ar[i] = nextInt();
			return ar;
		}

		public long nextLong () {
			if (!hasNext()) throw new NoSuchElementException();
			boolean minus = false;
			int temp = readByte();
			if (temp == '-') {
				minus = true;
				temp = readByte();
			}
			if (temp<'0' || '9'<temp) throw new NumberFormatException();
			long n = 0;
			while (isPrintable(temp)) {
				if ('0'<=temp && temp<='9') {
					n *= 10;
					n += temp - '0';
				}
				else throw new NumberFormatException();
				temp = readByte();
			}
			return minus? -n : n;
		}

		public long[] nextLongArray (int n) {
			long[] ar = new long[n];
			for (int i=0; i<n; i++) ar[i] = nextLong();
			return ar;
		}

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

		public double[] nextDoubleArray (int n) {
			double[] ar = new double[n];
			for (int i=0; i<n; i++) ar[i] = nextDouble();
			return ar;
		}

		public void close () {
			try {in.close();}
			catch(Exception e){}
		}

	}

}
