#include<iostream>

 #include<cstdio>

 using namespace std;

 const int maxn = +;

 int n,m,q;

 int p[maxn],r[maxn];

 int finds(int x)

 {

     if(p[x]==x)  return x;

     int tmp = p[x];

     p[x] = finds(p[x]);

     r[x] = r[x] + r[tmp];

     return p[x];

 }

 void unions(int a, int b, int x, int y, int s)

 {

     p[x] = y;

     r[x] = s+r[b]-r[a];

 }

 int main()

 {

     //freopen("in.txt","r",stdin);

     scanf("%d%d%d",&n,&m,&q);

     for(int i=;i<=n;i++)   {p[i]=i;r[i]=;}

     while(m--)

     {

         int a,b,s;

         scanf("%d%d%d",&a,&b,&s);

         int x = finds(a);

         int y = finds(b);

         unions(a,b,x,y,s);

     }

     while(q--)

     {

         int a,b;

         scanf("%d%d",&a,&b);

         int x = finds(a);

         int y = finds(b);

         if(x!=y)  puts("-1");

         else printf("%d\n",r[a]-r[b]);

     }

     return ;

 }