#include <iostream>
using namespace std;

int tree[10010] = {0};
int main() {
    int L, n;
    cin >> L >> n;
    for (int i=1;i<L+1;++i){
        tree[i] = 1;
    }
    int a,b;
    while(scanf("%d %d",&a,&b)!=EOF){
        for(int i=a; i<=b;++i){
            tree[i] = 0;
        }
    }
    int count = 1;
    for (int i=0; i<L+1;++i){
        if(tree[i]){
            count+=1;
        }
    }
    printf("%d\n",count);
}
// 64 位输出请用 printf("%lld")