#include <bits/stdc++.h>

using namespace std;

int n,m,k;

int x,y,z;

int Map[5005][5005],CFOfMap[5005][5005];

int Equal0(int X,int Y)
{
    return (Map[X-1][Y]+Map[X][Y-1]-Map[X-1][Y-1]+CFOfMap[X][Y]);
}

void SquareDO(int X,int Y,int DoNum)
{
    CFOfMap[X][Y]+=DoNum;
    CFOfMap[X+k][Y]-=DoNum;
    CFOfMap[X][Y+k]-=DoNum;
    CFOfMap[X+k][Y+k]+=DoNum;
}

void doit()
{

    int NeedTime=0;

    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=n;j++)
        {


            if((i+k>n+1||j+k>n+1)&&Equal0(i,j))
            {
                //cout<<Map[i-1][j]<<" "<<Map[i][j-1]<<" "<<Map[i-1][j-1]<<" "<<CFOfMap[i][j]<<endl;
                printf("-1");

                exit(0);
            }




            if(Equal0(i,j))
            {
                NeedTime+=abs(Equal0(i,j));

                SquareDO(i,j,-Equal0(i,j));
            }
            //cout<<Equal0(i,j)<<" ";
            Map[i][j]=Equal0(i,j);
        }

        //cout<<endl;

    }


    /*
    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=n;j++)
        {
            cout<<Equal0(i,j)<<" ";
        }
        cout<<endl;
    }
    */

    printf("%d",NeedTime);
}

void init()
{
    scanf("%d%d%d",&n,&m,&k);

    for(int i=1;i<=m;i++)
    {
        scanf("%d%d%d",&x,&y,&z);

        Map[x][y]=z;

    }

    for(int i=1;i<=n;i++)
    {
        for(int j=1;j<=n;j++)
        {
            CFOfMap[i][j]=Map[i][j]-Map[i-1][j]-Map[i][j-1]+Map[i-1][j-1];
            //cout<<CFOfMap[i][j]<<" ";
        }
        //cout<<endl;
    }


    doit();


}

int main()
{

    init();

    return 0;
}