#include <iostream>

int main(){
    int l(1<=1<=10000),m(1<=m<=100);
    int s=0;
    std::cin>>l,m;
    int arr[l + 1] = {0};

    int u, v;  

    for (int i = 1; i <= m; i++){

        std::cin >> u >> v;

        for(int j = u; j <= v; j++){
            if(arr[j] == 0){
                arr[j]=1;}
        }
    }

    for(int i = 0; i <= l; i++){
        if (arr[i] == 0){
            s++;}
    }
    std::cout<< s <<std::endl;
    return 0;
}