#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int N = 210;
int d[N][N];
int n,m,k;
int main()
{
    cin>>n>>m>>k;
    for (int i=1;i<=n;i++)
        for (int j=1;j<=n;j++)
        {
            if (i==j) d[i][j]=0;
            else d[i][j]=0x3f3f3f3f;
        }
    while (m--)
    {
        int a,b,c;
        cin>>a>>b>>c;
        d[a][b]=min(d[a][b],c);
    }
    for (int k=1;k<=n;k++)
        for (int i=1;i<=n;i++)
            for (int j=1;j<=n;j++)
                d[i][j]=min(d[i][j],d[i][k]+d[k][j]);
    while (k--)
    {
        int x,y;
        cin>>x>>y;
        if (d[x][y]>0x3f3f3f3f/2) puts("impossible");
        else cout<<d[x][y]<<endl;
    }
    return 0;
}