#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv[]) {
    int terminos = atoi(argv[1]);
    if (terminos <= 3) {
        printf("1\n");
        return 0;
    }
    unsigned long int a = 1;
    unsigned long int b = 1;
    unsigned long int c = 1;
    unsigned long int aux;
    for (int i = 4; i <= terminos; ++i) {
        aux = a + b + c;
        c = b;
        b = a;
        a = aux;
    }
    printf("%lu\n", a);
    return 0;
}