import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;


public class iSoar {
	static Scanner in=new Scanner(System.in);
	public static void main(String[] args) {
		while(true){
			double L = in.nextDouble();
			if(!(L>0)) break;
			ArrayList<Double> leftEndPoints = new ArrayList<Double>();
			ArrayList<Double> rightEndPoints = new ArrayList<Double>();
			leftEndPoints.add((double)0);
			rightEndPoints.add(L);
			while(true){
				double l = in.nextDouble();
				double r = in.nextDouble();
				if(l>r) break;
				int index1=-1;
				int index2=-1;
				for(int i=0;i<leftEndPoints.size();i++){
					if((l>leftEndPoints.get(i))&&(l<rightEndPoints.get(i))){
						index1=i;
					}
					if((r>leftEndPoints.get(i))&&(r<rightEndPoints.get(i))){
						index2=i;
					}
				}
				if(index1==-1){
					if(index2!=-1){
						leftEndPoints.set(index2, r);
					}
				}
				else{
					if(index2==-1){
						rightEndPoints.set(index1, l);
					}
					else{
						if(index1==index2){
							double val = rightEndPoints.get(index1);
							rightEndPoints.set(index1, l);
							rightEndPoints.add(index1+1, val);
							leftEndPoints.add(index1+1,r);
						}
						else{
							rightEndPoints.set(index1, l);
							leftEndPoints.set(index2,r);
							for(int j=index1+1;j<index2;j++){
								leftEndPoints.remove(j);
								rightEndPoints.remove(j);
							}
						}
					}
				}
				
			}	
			double totalSize=0;
			for(int i=0;i<leftEndPoints.size();i++){
				double size = rightEndPoints.get(i) - leftEndPoints.get(i);
				totalSize = totalSize + size;
			}
			System.out.printf("The total planting length is %.1f\n",totalSize);
		}
	}
}