#include <iostream> using namespace std; int main(){ int l,m = 0; cin>>l>>m; int v = 0; int a[l+1]; for (int i = 0;i <= l;i++){ a[i] = 1; } for (int i = 1;i <= m ;i++) //建地铁的区段 m段 { int s,b; cin>>s>>b; for (int j = s;j <= b;j++){ a[j] = 0; } } for (int i = 0;i <= l;i++){ if (a[i] == 1){ v++; } } cout << v; return 0; }