9 530792195 6
 0 0 10
 1944219055 454183506 1369298327
 914835 2742837 9234739
 10 10 0
 2 2 0
 0 0 1
 3 10 1007169359
 0 1 0
 1016450951 2 9
 0 2 10
 2147483648 0 2147483647
 92134834 23742837 92374738
 1 4 3
 382601556 881329640 791556039
 0 0 58
 5 10 6
 10 6 8
 6 8 10
 3 2 0
 1 4 4
 2147483630 2147483642 2147483610
 1 0 0
 18 67 5
 50606342 2 1134945035
 8 97 83
 2147483648 0 0
 92134834 23742837 92374737
 5 0 5
 772486757 1747374885 377299255
 1 0 1
 1 2 3
 0 3 2
 2147483648 2147483647 2147483648
 51 10 91
 9214834 2742837 9234739
 1 2 2147483648
 67 81 1
 92134834 23742837 92374739
 2 7 95
 3 0 2
 5 9 0
 1 10 2
 246543403 71853598 1504509195
 2147483648 2147483648 2147483648
 48 6 7
 2147483648 2147483648 0
 1358352906 27037371 1947040615

                                                       530792195
                                                                0
                                                       1944219055
                                                          2742837
                                                               10
                                                                2
                                                                0
                                                       1007169359
                                                                0
                                                       1016450951
                                                                2
 2147483648                                                         92374738
                                                                3
                                                        881329640
                                                                0
                                                               10
                                                                8
                                                                8
                                                                2
                                                                4
                                                       2147483630
                                                                0
                                                               67
                                                         50606342
                                                               97
                                                                0
                                                         92374737
                                                                5
                                                        772486757
                                                                1
                                                                3
                                                                2
 2147483648                                                               91
                                                          9234739
 2147483648                                                               67
                                                         92374739
                                                               95
                                                                2
                                                                9
                                                               10
                                                       1504509195
 2147483648                                                               48
 2147483648                                                       1947040615

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

public class P209B {

  int [] p = new int [3];

  boolean peace() {
    return (((p[0] == 0) && (p[1] == 0)) ||
            ((p[0] == 0) && (p[2] == 0)) ||
            ((p[1] == 0) && (p[2] == 0)));
  }

  public PrintWriter run() {
    p[0] = sc.nextInt();
    p[1] = sc.nextInt();
    p[2] = sc.nextInt();
      
    if (p[0] + p[1] + p[2] == 0) {
      pw.println(-1);
      return pw;
    }

    int count = 0;

    while (!peace()) {
      int i1, i2;
      if (p[0] == 0) {
        i1 = 1;
        i2 = 2;
      } else if (p[1] == 0) {
        i1 = 0;
        i2 = 2;
      } else if (p[2] == 0) {
        i1 = 0;
        i2 = 1;
      } else {
        if (p[0] < p[1]) {
          i1 = 0;
          i2 = (p[1] < p[2]) ? 1 : 2;
        } else {
          i1 = 1;
          i2 = (p[0] < p[2]) ? 0 : 2;
        }
      }

      int i3 = ((i1 + 1) ^ (i2 + 1)) - 1;
      int m = Math.min(p[i1], p[i2]);
      
      p[i1] -= m;
      p[i2] -= m;
      p[i3] += m;
      count += m;
    }

    pw.println(count);

    return pw;
  }

  private Scanner sc = new Scanner(new BufferedInputStream(System.in));
  private PrintWriter pw = new PrintWriter(new BufferedOutputStream(System.out));

  public static void main(String... args) throws Exception {
    new P209B().run().close();
  }
}
