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

public class Main {
    static final StdIn sc = new StdIn();
    static final PrintWriter out = new PrintWriter(System.out);
    static final int mod = 1000000007;

    public static void main(String[] args) {
        solve();
        out.close();
    }

    static void solve() {
        String s = sc.next();
        int cnt = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '1') {
                cnt++;
            }
        }
        out.println(cnt);
    }

    static class StdIn {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public StdIn() {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public StdIn(InputStream in) {
            try {
                din = new DataInputStream(in);
            } catch (Exception e) {
                throw new RuntimeException();
            }
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public String next() {
            int c;
            while ((c = read()) != -1 && (c == ' ' || c == '\n' || c == '\r'))
                ;
            StringBuilder s = new StringBuilder();
            while (c != -1) {
                if (c == ' ' || c == '\n' || c == '\r')
                    break;
                s.append((char) c);
                c = read();
            }
            return s.toString();
        }

        public String nextLine() {
            int c;
            while ((c = read()) != -1 && (c == ' ' || c == '\n' || c == '\r'))
                ;
            StringBuilder s = new StringBuilder();
            while (c != -1) {
                if (c == '\n' || c == '\r')
                    break;
                s.append((char) c);
                c = read();
            }
            return s.toString();
        }

        public int nextInt() {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do
                ret = ret * 10 + c - '0';
            while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        public long nextLong() {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do
                ret = ret * 10 + c - '0';
            while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }

        public double nextDouble() {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do
                ret = ret * 10 + c - '0';
            while ((c = read()) >= '0' && c <= '9');
            if (c == '.')
                while ((c = read()) >= '0' && c <= '9')
                    ret += (c - '0') / (div *= 10);
            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() {
            try {
                if (bufferPointer == bytesRead)
                    fillBuffer();
                return buffer[bufferPointer++];
            } catch (IOException e) {
                throw new RuntimeException();
            }
        }

        public void close() throws IOException {
            if (din == null)
                return;
            din.close();
        }
    }
}




Measured memory_bytes: 32456