为什么90分

#include <cstdio>
#include <iostream>
#define SM 4026000
#define INF 1e+6 

int n ;
int a[SM] , b[SM] ;
int cnt ;
int minn = INF , maxn = -INF ;
int main () {
    scanf ( "%d" , &n ) ; 
    for ( int i = 1 ; i <= n ; i ++ ) {
        int x , y ;
        scanf ( "%d%d" , &x , &y ) ;
        minn = std :: min ( x , minn ) ;
        maxn = std :: max ( y , maxn ) ;
        for ( int i = x ; i < y ; i ++ )
            a[i] = 1 ; 
    }
    for ( int i = minn ; i <= maxn ; i ++ )
        if ( a[i] ) cnt ++ ;
    std :: printf ( "%d" , cnt ) ;
    return 0 ;
}