> #include <iostream>
> #include <string.h>
> 
> using namespace std;
> 
> struct node
> {
> 	int x,y;
> 	int len;
> }num[20005];
> 
> int cmp(const void *a,const void *b)
> {
> 	
>     struct node*c = (node *)a;
>     struct node*d = (node *)b;
>     return d->len - c->len;
> }
> 
> int p[1008];
> 
> int Find_set(int x)
> {
> 	if(x != p[x])
> 	{
> 		p[x] = Find_set(p[x]);
> 	}
> 	return p[x];
> }
> 
> void Make_set(int x)
> {
> 	int i;
> 	for(i=0; i<x; i++)
> 	{
> 		p[i] = i;
> 	}
> }
> 
> int main()
> {
> 	__int64 sum;
> 	int N,M;
> 	int i;
> 	cin>>N>>M;
> 	for(i=0; i<M; i++)
> 	{
> 		cin>>num[i].x>>num[i].y>>num[i].len;
> 	}
> 	qsort(num,M,sizeof(num[0]),cmp);
> 	Make_set(N);
> 	sum=0;
> 	for(i=0; i<M; i++)
> 	{	
> 		if(Find_set (p[num[i].x]) != Find_set (p[num[i].y]))
> 		{
> 		
> 			sum += num[i].len;
> 			p[Find_set(num[i].x)] = p[Find_set(num[i].y)];
> 		}
> 	}
> 	printf("%I64d", sum);
> 	return 0;
> }




for(i=0; i<N-1; i++)
	{
		if(Find_set(p[i]) == Find_set(p[i+1]))
		{
			count++;
			continue;
		}
		else
			break;
	}
	if(count == N-1)
		printf("%I64d\n", sum);
	else
		printf("-1\n");