Saruman of Many Colours

problem statement is here

#include<stdio.h>

#include<string.h>

int main(){

long int a,b,t,i,j,sum,n,u,k;

scanf("%ld",&t);

while(t--){

j=1;

sum=0;b=0;u=0;

char ar[25000];

scanf("%ld %ld",&n,&k);

scanf("%s",ar);

for(i=0;i<n;i++){

if(ar[i]>96 && ar[i]<123){

if(ar[i]==u){

b++;

if(b==k){

j=0;

b=0;

u=0;

}

}else{

sum++;

u=ar[i];

b=1;

if(b==k){

j=0;

b=0;

u=0;

}

}

}

}

if(j==1){

printf("-1\n");

}else{

printf("%ld\n",sum);

}

}

return 0;

}