gistfile1.c

#include <stdio.h>
#define BUF_SIZE 256


int main()
{
    char A[BUF_SIZE], B[BUF_SIZE], C[BUF_SIZE];

    scanf("%s %s %s", &A, &B , &C);

    int base;

    long int c_A, c_B, c_C; // converted

    for(base = 2; base < 36; base++)
    {
        c_A =  strtol(&A, NULL, base);
        c_B =  strtol(&B, NULL, base);
        c_C =  strtol(&C, NULL, base);

        if((c_A + c_B) == c_C && (c_A != 0 && c_B != 0 && c_C != 0))
        {
            printf("%d", base);
            return 0;
        }
    }
    printf("-1");
    return 0;
}