自然而然

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

int main() {
    unsigned i, j, k = 0, n = 1e8;
    for (i = 0; i < n; i++)
        for (j = 0; j < RAND_MAX; j += rand())
            k++;
    printf("%f\n", (double)k / n);
}

$ gcc -O3 a.c && ./a.out
2.718219