#include<algorithm>

using namespace std;

bool l[10001];

int M,L,start,end;

int main(){

cin >> L >> M;

for (int i = 0; i < M; ++i){

cin >> start >> end;

for (int i = start; i <= end; ++i) l[i] = true;

}

cout << count(l, l + L + 1, false);

return 0;

}