#include<fstream.h>
int mat[16020][2],tt,n;
int gasit(unsigned long m)
{	unsigned long k,i;
	for(k=1;k<=n;k++)
		for(i=mat[k][1];i<mat[k][2];i++)
			if(i==m) return 1;
	return 0;
}

int main(void)
{	unsigned long min,max,i,tt,k,act;
	ifstream in("heavymetal.in");
	in>>n>>min>>max;
	mat[1][1]=min;
	mat[1][2]=max;
	for(i=2;i<=n;i++)
	{	in>>mat[i][1];
		if(mat[i][1]<min)min=mat[i][1];
		in>>mat[i][2];
		if(mat[i][2]>max)max=mat[i][2];
	}
	in.close();
	ofstream out("heavymetal.out");
	act=min+1;
	tt=2;
	while(act!=(max-1))
	{	if(gasit(act)) tt++;
		act++;
	}

	out<<tt;

	out.close();
	return 0;
}