C++ :

#include<iostream>
using namespace std;

int main(){
	int a[1000010] = {0};
	int n,m,i,x,y;
	cin>>n>>m;
	//标记开关(m次标记)
	for(i = 1;i <= m;i++){
		cin>>x>>y;
		a[x] = 1 - a[x];//取反
		a[y + 1] = 1 - a[y + 1];//结束位置要多标记一位
	} 
	
	int s = 0;
	int r = 0;//正面朝上的个数
	//存储结果数组
	for(i = 1;i <= n;i++){
		s = s + a[i];
		r = r + s % 2;
	}
	
	cout<<r<<endl;
	
}

Pascal :

var
a,f:array[1..10000000] of longint;
b,i,j,n,s,x,y:longint;
begin

read(b,n);
for i:=1 to n do
a[i]:=0;
for i:=1 to n do
begin
readln(x,y);
a[x]:=1-a[x];
a[y+1]:=1-a[y+1];
end;
for i:=1 to b do
begin
if a[i]=1
then s:=s+1;
f[i]:=s mod 2;
end;
s:=0;
for i:=1 to b do
if f[i]=1
then s:=s+1;
writeln(s);

end.