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

class Main implements Runnable {
	
	public static void main(String[] args) {
		new Main().run();
	}
	

	public void run() {
		FastScanner sc = new FastScanner();
		PrintWriter pw=new PrintWriter(System.out);

		int N=sc.nextInt();
		int D=sc.nextInt();
		int[] P=new int[N];
		int[] Q=new int[N];
		for (int i=0;i<N;++i) {
			P[i]=sc.nextInt();
			Q[i]=sc.nextInt();
		}
		long ok=-(long)1e8;
		long ng=1;
		long INF=Long.MAX_VALUE/3;
		while (ng-ok>1) {
			long m=(ok+ng)/2;
			long[] dp=new long[N];
			for (int d=0;d<D;++d) {
				long[] ndp=new long[N];
				Arrays.fill(ndp, -INF);
				int imax=0;
				for (int i=0;i<N;++i) if (dp[imax]<dp[i]) imax=i;
				for (int i=0;i<N;++i) {
					if (i!=imax&&dp[imax]-P[i]>=m) {
						ndp[i]=dp[imax]-P[i]+Q[i];
					}
				}
				for (int i=0;i<N;++i) {
					if (i!=imax&&dp[i]-P[imax]>=m) {
						ndp[imax]=Math.max(ndp[imax], dp[i]-P[imax]+Q[imax]);
					}
				}
				dp=ndp;
			}
			if (Arrays.stream(dp).max().getAsLong()>=m) {
				ok=m;
			} else {
				ng=m;
			}
		}
		pw.println(ok);
		pw.close();
	}
	
	void tr(Object...objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}



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;}
    public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; 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 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() {
        long nl = nextLong();
        if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
        return (int) nl;
    }
    public double nextDouble() { return Double.parseDouble(next());}
}