求助站外题

#include <cstdio>
#include <cstring>

bool isPrime[1000000010];
long long Prime[60000010], cnt = 0;

void GetPrime(long long n)
{
    memset(isPrime, 1, sizeof(isPrime));
    isPrime[1] = 0;

    for(long long i = 2; i <= n; i++)
    {
        if(isPrime[i]) 
            Prime[++cnt] = i; 
        for(long long j = 1; j <= cnt && i*Prime[j] <= n; j++) 
        {
            isPrime[i*Prime[j]] = 0;

            if(i % Prime[j] == 0)
                break; 
        }
    }
}

int main()
{
    long long n=1000000000;
    GetPrime(n);
        long long k;
        scanf("%lld",k);
        long long c=2;
            c=2;
            long long m=Prime[k];
        while(m-Prime[c]!=0)
        {
            if(m-Prime[c]<0)
            {
                c=-1e9;break;
            }
            c++,m-=Prime[c];
        }
        if(c!=-1e9)
        printf("%lld\n", c);

    return 0;
}