eq.c

#include <stdio.h>
#include <assert.h>
#include <math.h>

#define INPUT "input.txt"
#define OUTPUT "output.txt"

int sumOfDigits(int x) {
    int res = 0;

    do {
        res += (x % 10);
        x /= 10;
    } while(x);

    return res;
}

int findSol(unsigned long long n) {
    int x, min = -1;

    for(x = 0; x < 180; x++) {
        if((x*x + sumOfDigits(x)*x - n) == 0) {
            min = x;
            break;
        }
    }

    return min;
}

int main(int argc, char **argv) {
    FILE *file_in, *file_out;
    unsigned long long n;

    file_in = fopen(INPUT, "r");
    file_out = fopen(OUTPUT, "w");

    assert(fscanf(file_in, "%llu", &n) == 1);
    fprintf(file_out, "%d", findSol(n));

    fclose(file_in);
    fclose(file_out);

    return 0;
}