题解 | [NOIP2015]金币

#include <stdio.h>

int main() {
    int k,days=0,sum=0,i=1;
    scanf("%d",&k);
    while(1){
        days += i;
        if(days>k){
            days -= i;
            break;
        }
        sum += i*i;
        i++;
    }
    int left = k-days;
    printf("%d\n",sum+i*left);
    return 0;
}