C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	//有树标记为0，没有树标记为-1 
	int l,m,a[10010] = {0},i,x,y,c = 0,j;
	cin>>l>>m;
	for(i = 1;i <= m;i++){
		cin>>x>>y;
		for(j = x;j <= y;j++){
			if(a[j] == 0){
				a[j] = -1;
				c++;	
			}
		}
	}
	
	cout<<l+1-c;
}

Pascal :

var a,b,c:array[0..10000] of longint;
    l,m,i,j,k,s:longint;
begin
read(l,m);
for i:=0 to l do
a[i]:=1;
for i:=1 to m do
read(b[i],c[i]);
for j:=1 to m do
for k:=b[j] to c[j] do
a[k]:=0;
for i:=0 to l do
s:=s+a[i];
write(s);
end.

Java :

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int num1 = scanner.nextInt();
        int num2 = scanner.nextInt();

//        String str = new String();
        int[] a = new int[num2 * 2];
        int[] b = new int[num1+1];
        double s1 = 0,s2 = 0,avg1,avg2;
        int[] count = new int[10000];
        for (int i = 0; i < num2 * 2; i++) {
            a[i] = scanner.nextInt();
        }
        for (int i = 0; i <= num1; i++) {
            b[i] = 1;
        }
        int j ,k;
        for (int i = 0; i < num2 * 2; i+=2) {
            j = a[i];
            k = a[i + 1];
            for(int m = j;m <= k;m++){
                b[m] = 0;
            }
        }
        int x = 0;
        for (int i = 0; i <= num1; i++) {
            if(b[i] != 0)x++;
        }
        System.out.println(x);


    }public static void paiXu(int[] a,int length){
        int temp;
        for(int i = 0;i < length - 1;i++){
            for(int k = 0;k < length - i - 1;k++){
                if(a[k] > a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    } public static void paiXud(int[] a){
        int temp;
        for(int i = 0;i < a.length - 1;i++){
            for(int k = 0;k < a.length - i - 1;k++){
                if(a[k] < a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    } public static void paiXux(int[] a){
        int temp;
        for(int i = 0;i < a.length - 1;i++){
            for(int k = 0;k < a.length - i - 1;k++){
                if(a[k] > a[k + 1]){
                    temp = a[k];
                    a[k] = a[k + 1];
                    a[k + 1] = temp;
                }
            }
        }
    }

    public static int MAX(int[] a){
        int max = a[0];
        for (int i = 0; i < a.length; i++) {
            if(max < a[i]){
                max = a[i];
            }
        }
        return max;
    }

    public static int MIN(int[] a){
        int min = a[0];
        for (int i = 0; i < a.length; i++) {
            if(min > a[i]){
               min = a[i];
            }
        }
        return min;
    }
    public static int sum(int a){
        int x,s = 0;
        while(a != 0){
            x = a % 10;
            s = s + x;
            a /= 10;
        }
        return s;
    }
    public static boolean isPrime(int a){
        if(a <= 1)return false;
        else
        for(int i = 2;i <= Math.sqrt(a);i++){
            if(a % i == 0)return false;
        }return true;
    }

    public static int qiuYu(int i){
        int a;
        a = i % 10;
        return a;
    }
}

Python :

ml=list(map(int,input().split()))
ml[0]=ml[0]+1
bl=[]
for i in range(ml[1]):
    tl=list(map(int,input().split()))
    bl.append(tl)
list=[]
for i in range(ml[0]):
    list.append(1)
for i in bl:
    list[i[0]]=0
    for j in range(i[1]-i[0]):
        list[j+i[0]+1]=0
t=0
for i in range(len(list)):
    if list[i]==1:
        t+=1
print(t)