include <bits/stdc++.h>

using namespace std; int a[10005]; int main() {

int l, m, c, d;
cin >> l >> m;

for (int i = 0; i < m; i++) {
	cin >> c >> d;
	for (int j = c; j <= d; j++) {
		a[j] = 1;
	}
}	
int b = 0;
for (int e = 0; e < l + 1; e++) {
	if (a[e] == 0) {
		b++;			
}
	}

cout<<b; return 0; }