校门外的树

#include<stdio.h>
int main()
{
int tree[10001];
int l, m, start, end, i, j, total = 0;

scanf("%d %d", &l, &m);

for( i = 0; i <= l; i++ )
{
tree[i] = 1;
}

for( i = 1; i <= m; i++ )
{
scanf("%d %d", &start, &end );

for( j = start; j <= end; j++ )
{
tree[j] = 0;
}
}

for( i = 0; i <= l; i++ )
{
total = total + tree[i];
}

printf("%d\n", total);

return 0;

}