C++ :

#include <iostream>
#include <cmath>
using namespace std;
int l,m,s[101],t[101],left,right,sum=0;
int main()
{
    cin>>l>>m;
    for (int i=1;i<=m;i++) cin>>s[i]>>t[i];
    for (int j=2;j<=m;j++)
    {  
        for (int i=m;i>=j;i--) 
        {
          if (s[i]<s[i-1]) 
          {s[0]=s[i],s[i]=s[i-1],s[i-1]=s[0],
          t[0]=t[i],t[i]=t[i-1],t[i-1]=t[0];}
        }
    }
//    for (int i=1;i<=m;i++) cout<<s[i]<<" "<<t[i]<<endl;
    int left=s[1]; int right=t[1];
    for (int i=2;i<=m;i++)
    {
        if (s[i]<=right) {right=max(right,t[i]);}
        else 
        {sum+=right-left+1; left=s[i]; right=t[i];}
    }
    sum+=right-left+1;
    cout<<l-sum+1<<endl;
//    system ("pause");
    return 0;
}

Pascal :

type
 rec=record
  x,y,l,r,c:longint;
 end;
var
 ans,l,m,i,x,y,tot:longint;
 tree:array[0..40000] of rec;
procedure make(l,r:longint);
var
 now:longint;
begin
 inc(tot);
 now:=tot;
 tree[now].x:=l;
 tree[now].y:=r;
 if l+1<=r then
 begin
  tree[now].l:=tot+1;
  make(l,(l+r) div 2);
  tree[now].r:=tot+1;
  make((l+r) div 2+1,r);
 end;
end;
procedure insert(v,x,y:longint);
begin
 if (x<=tree[v].x)and(tree[v].y<=y) then
 begin
  inc(tree[v].c);
  exit;
 end;
 if x<=(tree[v].x+tree[v].y) div 2 then insert(tree[v].l,x,y);
 if y>(tree[v].x+tree[v].y) div 2 then insert(tree[v].r,x,y);
end;
procedure find(v:longint);
begin
 if v=0 then exit;
 if tree[v].c>0 then
 begin
  inc(ans,tree[v].y-tree[v].x+1);
  exit;
 end;
 find(tree[v].l);
 find(tree[v].r);
end;
begin
 readln(l,m);
 make(1,l+1);
 for i:=1 to m do
 begin
  readln(x,y);
  insert(1,x+1,y+1);
 end;
 find(1);
 //writeln(ans,' ',l);
 writeln(l+1-ans);
end.

Python :

# coding=utf-8
l,m=map(int,input().split())
a=[]
s=["true"]*(l+1)
n=0
for i in range(m):
    b=list(map(int,input().split()))
    a.append(b)

for i in range(m):
    for j in range(a[i][0],a[i][1]+1):
        s[j]="false"

for k in s:
    if k=="true":
        n+=1
print(n)