import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.BufferedOutputStream;
import java.io.UncheckedIOException;
import java.nio.charset.Charset;
import java.util.StringTokenizer;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.BufferedReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author mikit
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        LightScanner in = new LightScanner(inputStream);
        LightWriter out = new LightWriter(outputStream);
        EDividingChocolate solver = new EDividingChocolate();
        solver.solve(1, in, out);
        out.close();
    }

    static class EDividingChocolate {
        public void solve(int testNumber, LightScanner in, LightWriter out) {
            int h = in.ints(), w = in.ints(), k = in.ints();
            char[][] s = in.chars(h);
            int ans = h + w - 2, all = 1 << (h - 1);
            int[] dest = new int[h], count = new int[h];
            outer:
            for (int i = 0; i < all; i++) {
                int res = 0;
                count[0] = 0;
                for (int j = 0; j < h - 1; j++) {
                    dest[j + 1] = dest[j];
                    if (((i >> j) & 1) == 1) {
                        count[++dest[j + 1]] = 0;
                        res++;
                    }
                }
                for (int j = 0; j < w; j++) {
                    boolean ok = true;
                    for (int l = 0; l < h; l++) {
                        if (s[l][j] == '0') continue;
                        if (k < ++count[dest[l]]) {
                            ok = false;
                            break;
                        }
                    }
                    if (ok) continue;
                    res++;
                    for (int l = 0; l <= dest[h - 1]; l++) count[l] = 0;
                    for (int l = 0; l < h; l++) {
                        if (s[l][j] == '0') continue;
                        if (k < ++count[dest[l]]) continue outer;
                    }
                }
                ans = Math.min(ans, res);
            }
            out.ans(ans).ln();
        }

    }

    static class LightWriter implements AutoCloseable {
        private final Writer out;
        private boolean autoflush = false;
        private boolean breaked = true;

        public LightWriter(Writer out) {
            this.out = out;
        }

        public LightWriter(OutputStream out) {
            this(new OutputStreamWriter(new BufferedOutputStream(out), Charset.defaultCharset()));
        }

        public LightWriter print(char c) {
            try {
                out.write(c);
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter print(String s) {
            try {
                out.write(s, 0, s.length());
                breaked = false;
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
            return this;
        }

        public LightWriter ans(String s) {
            if (!breaked) {
                print(' ');
            }
            return print(s);
        }

        public LightWriter ans(int i) {
            return ans(Integer.toString(i));
        }

        public LightWriter ln() {
            print(System.lineSeparator());
            breaked = true;
            if (autoflush) {
                try {
                    out.flush();
                } catch (IOException ex) {
                    throw new UncheckedIOException(ex);
                }
            }
            return this;
        }

        public void close() {
            try {
                out.close();
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
        }

    }

    static class LightScanner implements AutoCloseable {
        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;

        public LightScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
        }

        public String string() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new UncheckedIOException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public char[] chars() {
            return string().toCharArray();
        }

        public char[][] chars(int h) {
            char[][] res = new char[h][];
            for (int i = 0; i < h; i++) res[i] = chars();
            return res;
        }

        public int ints() {
            return Integer.parseInt(string());
        }

        public void close() {
            try {
                this.reader.close();
            } catch (IOException ex) {
                throw new UncheckedIOException(ex);
            }
        }

    }
}



Measured memory_bytes: 34904