import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.NoSuchElementException;


public class Main{
	static FastScanner sc = new FastScanner();
	static PrintWriter out = new PrintWriter(System.out);
	//static PriorityQueue <Integer> q ;
	//static Deque <Integer> q;
	//Deque deque = new LinkedList();
	//static HashMap <Integer, Integer> map;
	//static int n;
	static final int mod = 1_000_000_007;
	static int H;
	static int W;
	//static final int[] dx = {0,1,0,-1};
	//static final int[] dy = {-1,0,1,0};
	//static int max_d;
	static int [][] S;				
	
	public static void main(String[] args) {
		H = sc.nextInt();
		W = sc.nextInt();
		int k = sc.nextInt();
		
		S = new int [W+1][H+1];
		for (int i=1; i<=H; i++) {
			String s = sc.next();
			for (int j=1; j<=W; j++) {
				S[j][i] = s.charAt(j-1)-'0';
			}
		}
		
		int ans = Integer.MAX_VALUE;
		
		for(int i=1 ; i<(1 << H); i++) {
			int h_cut = 0;
			int group [] = new int [H+2];
			
			for(int j=1; j<=H; j++) {
				if((i >> j & 1) == 1 ) {
					group[j+1] = group[j]+1;
					h_cut++;
				}
				
				else 
					group[j+1] = group[j];	
			}
			
			int wcut = 0;
			boolean check = true;
			int [] white = new int [h_cut+1];
			
			for(int j=1; j<=W; j++) {
				boolean ok = true; 
				int col_count [] = new int [h_cut+1];
				
				for(int l=1; l<=H; l++) {
					white[group[l]] += S[j][l];
					col_count[group[l]] += S[j][l];
					
					if(white[group[l]] > k) 
						ok = false;
					
					if(col_count[group[l]] > k) 
						check = false;
				}
					
				if (!ok) {
					wcut++;
					white = col_count;
				}
			}
			
			if (check) 
				ans = Math.min(ans, h_cut+wcut);
		}
		out.println(ans);
		out.flush();
	}
	
	
	

	
	static class FastScanner {
	    private final InputStream in = System.in;
	    private final byte[] buffer = new byte[1024];
	    private int ptr = 0;
	    private int buflen = 0;

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

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

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

	    private void skipUnprintable() {
	      while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++;
	    }

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

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

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

	    public int nextInt() {
	      return (int) nextLong();
	    }

	    public int[] nextIntArray(int N, boolean oneBased) {
	      if (oneBased) {
	        int[] array = new int[N + 1];
	        for (int i = 1; i <= N; i++) {
	          array[i] = sc.nextInt();
	        }
	        return array;
	      } else {
	        int[] array = new int[N];
	        for (int i = 0; i < N; i++) {
	          array[i] = sc.nextInt();
	        }
	        return array;
	      }
	    }

	    public long[] nextLongArray(int N, boolean oneBased) {
	      if (oneBased) {
	        long[] array = new long[N + 1];
	        for (int i = 1; i <= N; i++) {
	          array[i] = sc.nextLong();
	        }
	        return array;
	      } else {
	        long[] array = new long[N];
	        for (int i = 0; i < N; i++) {
	          array[i] = sc.nextLong();
	        }
	        return array;
	      }
	    }
	  }
}


Measured memory_bytes: 45340