It is confirmed that these sections do not overlap with each other.
一句话 就变成水题了，，，
// by SiriusRen
#include <cstdio>
using namespace std;
int n,m,xx,yy;
int main(){
while(scanf("%d%d",&n,&m)&&(n||m)){
n++;
for(int i=1;i<=m;i++)scanf("%d%d",&xx,&yy),n-=(yy-xx+1);
printf("%d
",n);
}
}