萌新刚学OI，线段树90分求助！

qw，WA#1了。。

代码：

#include <stdio.h>

#define MAXN 16384

struct TREE_NODE{
    int l,r;
    int value;
} tree[MAXN << 2];

int n,m;
int a[MAXN];

inline void BUILD(int p,int l,int r){
    tree[p].l = l;tree[p].r = r;
    if(l == r){
        tree[p].value = 1;
        return ;
    }
    int mid = l+r >> 1;
    BUILD(p << 1,l,mid);
    BUILD(p << 1|1,mid+1,r);
    tree[p].value = tree[p << 1].value+tree[p << 1|1].value;
}

inline void UPDATE(int p,int l,int r){
    int &x = tree[p].l;int &y = tree[p].r;
    if(x == y){
        tree[p].value = 0;
        return ;
    }
    int mid = x+y >> 1;
    if(l <= mid)    UPDATE(p << 1,l,r);
    if(r >  mid)    UPDATE(p << 1|1,l,r);
    tree[p].value = tree[p << 1].value+tree[p << 1|1].value;
}

inline int SEARCH(int p,int l,int r){
    int &x = tree[p].l;int &y = tree[p].r;
    if(x >= l && y <= r)    return tree[p].value;
    int mid = x+y >> 1;
    int step = 0;
    if(l <= mid)    step += SEARCH(p << 1,l,r);
    if(r >  mid)    step += SEARCH(p << 1|1,l,r);
    return step;
}

signed main(){
    scanf("%d%d",&n,&m);
    BUILD(1,1,n);
    while(m --){
        int l,r;
        scanf("%d%d",&l,&r);
        UPDATE(1,l,r);
    }printf("%d",SEARCH(1,1,n)+1);

    return 0;
}

(=・ω・=)