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;
}

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)