import java.util.*;
public class b {
	Scanner in=new Scanner(System.in);
	public static void main(String[] args) {
		new b().go();
	}
	private void go() {
		while(true){
			double l=in.nextDouble();
			if(l<=0)break;
			PriorityQueue<Double>s=new PriorityQueue<Double>(),f=new PriorityQueue<Double>();
			while(true){
				double a=in.nextDouble(),b=in.nextDouble();
				if(a>b)break;
				s.add(Math.max(a, 0));
				f.add(Math.max(b,0));
			}
			double ans=0;
			double loc=0;
			while(loc<l&&!s.isEmpty()){
				double ns=s.poll();
				ans+=Math.min(ns,l)-loc;
				int open=1;
				while(open!=0){
					if(!s.isEmpty()&&s.peek()<f.peek()){
						s.poll();
						open++;
					}else{
						loc=f.poll();
						open--;
					}
				}
			}
			if(loc<l)ans+=l-loc;
			System.out.printf("The total planting length is %.1f\n",ans);
		}
	}
}
//I-Soar
//building hedges to block the highway
//priority queue treeset