import java.util.Collections;
import java.util.Scanner;
import java.util.ArrayList;

public class ISoar {
	Scanner in = new Scanner(System.in);
	
	ArrayList<Building> buildings;
	
	void start() {
		while (true) {
			float L = in.nextFloat();
			// End-of-input condition
			if (L <= 0) return;
			// Input test case
			buildings = new ArrayList<Building>();
			while (true) {
				float x1 = in.nextFloat();
				float x2 = in.nextFloat();
				// End-of-test case condition
				if (x1 > x2) break;
				buildings.add(new Building(x1, x2));
			}
			// Sort buildings by increasing left endpoint
			Collections.sort(buildings);
			// Merge redundant intervals
			merge();
			// Calculate uncovered distance
			for (int i = 0; i < buildings.size(); i++) {
				L -= buildings.get(i).dx;
			}
			// Print desired output
			System.out.printf("The total planting length is %.1f\n", L);
		}
	}
	
	void merge() {
		// List with only validated intervals
		ArrayList<Building> mergedList = new ArrayList<Building>();
		// Initialize intervals to consider
		float left = buildings.get(0).xL;
		float right = buildings.get(0).xR;
		for (int i = 1; i < buildings.size(); i++) {
			float cur_left = buildings.get(i).xL;
			float cur_right = buildings.get(i).xR;
			// End condition for last interval considered
			if (i == buildings.size()-1) {
				mergedList.add(new Building(left, right));
				break;
			}
			// Merge intervals if necessary
			if (cur_left <= left && cur_right > right) {
				right = cur_right;
			}
			// Otherwise, add verified interval to merged list
			else {
				mergedList.add(new Building(left, right));
				left = cur_left;
				right = cur_right;
			}
		}
		// Replace original list with merged list
		buildings = mergedList;
	}
	
	// Class for each building
	class Building implements Comparable<Building> {
		// Left endpoint
		float xL;
		// Right endpoint
		float xR;
		// Length of building
		float dx;
		// Constructor
		Building(float a, float b) {
			xL = a;
			xR = b;
			dx = b - a;
		}
		// For sorting (will sort by increasing xL)
		public int compareTo(Building other) {
			float starts = this.xL - other.xL;
			if (starts > 0f) return 1;
			else if (starts < 0f) return -1;
			else return 0;
		}
	}
	
	public static void main(String[] args) {
		new ISoar().start();
	}
}