【CodeForces 622A】Infinite Sequence

#include<cstdio>
int main()
{
    long long n,i;
    scanf("%lld",&n);
    for(i=1; i<=1e8; i++)
    {
        if(i*(i+1)/2>=n) break;
    }
    printf("%lld",n-i*(i-1)/2);
    return 0;
}

#include<cstdio>
int main()
{
    long long n,i;
    scanf("%lld",&n);
    i=1;
    while(i*(i+1)/2<n)
    {
        i++;
    }
    printf("%lld",n-i*(i-1)/2);
    return 0;
}

#include<cstdio>
#include<cmath>

int main()
{
    long long n,k;

    scanf("%lld",&n);
    k=floor(sqrt(2*n));
    if (k*(k+1)/2<n)
    {
        k++;
    }
    printf("%lld",n-k*(k-1)/2);
    return 0;
}