[백준] 1790번 : 수 이어 쓰기 2 - C

https://www.acmicpc.net/problem/1790

#include<stdio.h>
#include<string.h>
#include <math.h>
static long count(long t){
    long num=1;
    for(int i=0;i<t;i++){
        num*=10;
    }
    return num;
}
int main() {
    int n,k;
    scanf("%d %d",&n,&k);
    int t=1;
    long num=0;
    int r=0;
    int e=0;
    while(1){
        long max=(count(t)-count(t-1))*t;
        
        if(num+max>=k){
            k-=num;
            r=k/t;
            e=k%t;
            n-=r;
            break;
        }
        num+=max;
        n-=count(t)-count(t-1);
        t++;
    }
    r+=count(t-1)-1;
    if(n<0 || n==0 && e!=0) printf("-1");
    else if(e==0){
        char m[10];
        sprintf(m,"%d",r);
        printf("%c",m[strlen(m)-1]);
    }
    else{
        r+=1;
        char m[10];
        sprintf(m,"%d",r);
        printf("%c",m[e-1]);
    }
}